Well, the essentials to drive it away from the garage are head gasket/bolts, the belt and tensioner, auxiliary belt or belts (dependent upon model),
filter, thermostat and coolant. If you can get a new water pump for £17 it seems a false economy not to do so. That lot cost me around £155. If the cylinder head is found to be 'true' but there's no obvious damage to the gasket then you should get it pressure tested to ensure there's not an internal crack (£35 ex VAT for me London). You can do the second
So let's say £200 for parts/testing, allowing for a garage getting stuff a bit cheaper but adding a bit of mark-up.
The 'book hours' for the job are 5.85, so you can try to agree a fixed labour fee based on that. In London I can't find anything under £45ph, so that would be £270 labour. My suspicion is that it really can't be done in that time, unless you're a Renault technician who's done precisely the same procedure once a week for the last three years. I'd be pleasantly surprised if it came in at much under ten hours at a regular garage.
So... £200 parts, £400 labour, + VAT: £700. Cost to you: £350. A little less 'out of town'.
Assuming you have all necessary tools and equipment the job will cost you £200.
At best a £150 saving. Unless you value your time at less than a fiver an hour, I'd say bite your gentleman's hand off.
Do try and establish exactly what the garage propose to replace, though...
Of course an alternative, if you're feeling brave, would be to ask your gentleman for a £350 contribution and then do it yourself.