I'm assuming, given where you've posted this, that it is the dreaded EGR valve issue (your post isn't specific)?
In which case, given the mileage of the car, the fact that it's a known
problem with VAG EGRs and you have full dealer service history, I would think you could maybe get a bit more good will from Seat. There are various posts about this issue and differing experiences in terms of good will received. A bit more wrangling, if you can be bothered, might save you a few more quid. The only thing counting against you is the age of the car, i.e. now out of warranty.
Just be thankful that the
problem has arisen now, and not say when you've done 60000 miles, the car is 6 years old and you haven't had it dealer serviced. Because then you're looking at a bill of £600 to £1000!
The EGR on the MkV
Ibiza is a pig to get to which bumps up the labour charge, not sure about the Altea, but if £250 is based on 50% seat contribution then it sounds like it might be a similarly nasty job.
You also don't say whether it's a diesel or petrol, but general wisdom around here says that using a good quality fuel, i.e. not supermarket stuff, might
help prevent the issue re-occurring again in the future.
Best of luck!