Advice needed please.

Ok this is a bit long winded but need to tell you the facts but i will keep it to the point.

As you all know i had TDI's do the work on my car where in short they fitted the FI kit and rebuilt the engine with cosworth parts including rods, pistons,heads, cams etc etc.
From only a month i started using a bit of oil which eventually ended up over time as 5 litres every 1000 miles. In between this TDI's took my car back in to try and sort the problem out a few times. They upgraded the oil catch can system, they sent away my turboes to have them re-sealed etc,they changed the stock heads to Cosworth heads, they put Viton coated stem seals in and they stripped the engine down again and replaced the piston rings but all these things did not solve the problem.
When TDI's told me they had done as much as they could with the car and they wasnt willing to put any more free labour into it (i paid for the parts)i felt sick after all the money and effort i had put into the car. Not to be defeated i started self learning myself about the parts etc in my car when i came across some spec on the Cosworth pistons, Cosworth said with the ally material used in the making of them they deliberately make them so they can expand under high temperatures which prevents tiny cracks on extreme use however it is normal for slight piston slap on cold starts. They went on to say it is important that the cylinder to sleeve wall tolerances should be within 0.08 and 0.12 (so 0.1 would be perfect). Without telling this to TDI i went to them and in a round about way asked them what tolerances they had built the engine to and they told me they always build there engines to 0.05 to prevent piston slap etc.
I then wrote to cosworth and asked what problems this would cause and they emailed me back and said the toleraces of my engine where far to tight and under high power the pistons would of had no room to expand and would have pushed the rings into the sleeve walls causing ware.
I gave this info to TDI and they agreed to strip the engine again,get it re-bored and fit the slightly bigger Cosworth pistons along with all the other stuff which goes with doing this at the 0.1 tolerance Cosworth stated (by the way the instructions and tolerances are clearly shown on the instructions in every box,they just never read them).
I have now had my car back and done 2k miles and am pleased to say havn't used a drop of oil.

So after reading the facts what do you think i should do. TDI's sent me a bill for around 12K which they believe is half of the total cost of getting my car sorted including parts and all the initial investigation works as noted above. I have paid them 2k plus the cost of Cosworth heads a while back and was thinking of knocking them for the other 10K, trouble is they are saying if i dont pay them they will take me to court and remove their so called goodwill gesture of charging me for only 50% and claim the whole 20K instead.
Bare in mind the origional build i paid in full to them a couple of years back so this is just the amount they are c laiming to sort the oil problem out. There only excuse is that they can never give a warranty on this kind of build.

What do i do

.

I think they should be liable Tom,as it was their suggestion to build the engine with those parts,so in actual fact they should have been aware of the expansion of the pistons from day one.I know highly modified race builds carry no warranty as such,but in this case they used ill matching components,and tolerances of bores etc should be down to them.......have you thought about legal advice Tom......a Solicitors letter can often carry some clout......This is not the first issue you have had with this company....i remeber once when we were at the pod and they had left an old vacuum line port open which was sucking in unmetered air...!!!
I think you have a strong case....don't let them get away with it mate.....it's no different than you fitting a kitchen dude and getting it wrong,then when the customer complains you tell them that because it's a custom install and not laid out like it was in the showroom you can't guarentee it,or put it right.....
These tuners are often alike they wan't the work but more often do not stand by it.......if they can't complete a project properly they should be honest from the start and decline the build due to lack of knowledge and confidence in their abilities......
Don't let them screw you Tom.......
Get Cosworth to send you as much info as you can.....it may help

i agree with mr g i would get some legal advise but i wouldnt pay its there mistake at the end of the day

Yeah thanks for the reply lads, its pretty much what i thought as well, i just didn't want the aggro of a long legal battle at my time of life (middle aged )

Trouble is this wont be a small claims court as its over £5k, i just hope they get cold feet when they think about the evidence i have.

true. wouldn't stand for that, its a blatant mistake on their behalf.

Go for it Tom......you have a good case....

I would pursue legal advice Tom, see what your chances are legally. Then decide whether to go for it, sounds like you have a case though. Don't forget the power of the media, a lot of tuners rely on forums and good press for their business, if they know they are facing a high profile fight due to their incompetence they might think twice about pursuing it. I wouldn't threaten them but i would make them aware that the story will be documented honestly on select forums etc..

I hope you get a good outcome Tom, they shouldn't get away with this.