Don't think you'll get too far with the warranty company. Why not do it yourself? nice easy job.

The obvious one is 'seizure'.

I'm not familiar with the Corsa layout but such an item appears to cost between £15 and £25 after a simple ebay search, and should take a DIYer (you?) maybe half an hour to fit, a further £15 would buy 5 litres of 5 year concentrated coolant which is enough to renew the coolant on 2 small/medium cars.

Well I agree with you that a warranty on an 8 year old car was a waste of money. Bad call.

I'm assuming the thermostat housing is being replaced because the thermostat itself needs replacing and it is supplied as a complete unit. A replacement thermostat on a car of this age is to be expected, so I wouldn't expect a warranty to cover it. If the thermostat issue has been present since you bought the car, then try the dealer you purchased the car from or are they hiding behind the warranty?

Any idea of what I can claim as the cause of failure which is credible but at same time cant be excluded by the crooks at the warranty company.

Something coming to the end of it's life would never come under a warranty - no 'crooks' involved.