Fuel cost France
 

Im due to fly to Annemasse from the UK via Troyes, from Elstree. I'm flying a c152, and have estimated fuel costs of around £350-£400 one way.
Am I on the right lines?

C.A

Depends on your groundspeed.......And how much you lean, oh, and your routing, altitude (which may well affect how much head or tail wind you get) , power settings, etc.
Price per litre is currently between 2.15€ and 2.30€
If you have a Total card you can usually do self service which is a lot quicker

Avgas is €2.50, plus or minus 10%, in most of the EU, until you get to say Greece where it starts to go up a bit.

For the fuel burn you need to consult the POH where it should be stated for a given power setting, etc.

Can a C152 make Troyes from the UK?

That sounds a bit pessimistic. One way it should be about 4.5 hours flying in a C152. Let's say 100 KTAS and 25 l/hr; so about 110 litres or about £220 give or take.

And you can also claim a duty drawback on the fuel in the tanks at the start of the trip, which is about 37p/litre.