Blimey - I tell you what if that is the offer I would definitely do the water pump at the same time - the cost of the pump will be what, £40 quid tops from main dealer?
That gets you fresh antifreeze too and a flush as they will have to drain the cooling system down to do the pump - well worth it for peace of mind.
I have often heard that it is often not the timing belt itself that snaps, it is more often an ancillary component that is run off the belt - eg the water pump.
I have known the pump bearings to break up, probably due to silting up and this causes the belt to slip off.
If anyone goes with this pls post how it went/cost/downtime - also if Seat main dealer or VW or independant?
Cheers chaps