Valve cover leak or head gasket?

I wonder if someone can help. I bought an 05 Ford KA on Saturday. The seller told me it was overheating and needed the thermostat changing. I drove it home (approx 2 miles) it was badly over heating. I left it parked up until Tuesday when I got a mobile mechanic to replace thermostat. He replaced it charged me £90 then told me the head gasket was gone and I may as well scrap the car. I have since seen on the internet that it may only be the valve cover but how would I know? I really don't have the money to take it to another garage to find out. I have read that there maybe stuff in the oil, but the oil has recently been changed and so there's nothing in there, the mechanic gave his reasons as the fan was cutting in too quickly when the engine was running but stationery, (although he was revving it high and for long periods) also that the pipes were getting too hot too quickly. Any advice would be greatly appreciated.

A valve cover would NEVER cause overheating. You pretty much got screwed here by ignoring Rule #1 of buying a used car.

Read this

http://ricksfreeautorepairadvice.com/used-car-checklist/

Open the radiator cap and pump air into each cylinder. If you see pocket airs come up in there then that means you have a leak head gasket. It would cause your car to overheat.
If you don't have an air compressor, still open the radiator cap and start your engine up. If you see big pocket airs come up in there then that means you have a leak head gasket, or cracked cylinder walls.
The valve cover has nothing to do with your car overheating.

Did it not cross your mind that when the seller told you the stat need replacing why hadn't he done this simple job himself? As a result of your swallowing his BS you have bought trouble. You can cast around for all the alternatives you wish but I believe your mechanic has already told you the truth. If the owner changed the oil recently it was probably to get rid of the water in the oil. You could search the net for a firm to supply and fit an exchange head which is the best option. But it depends how much you wish to spend on an old car.

A valve cover leak will not cause overheating. It is the headgasket or other part of the cooling system.

If you need to top off your coolant level every day or two, then it is a head or intake manifold gasket.

The valve cover only deals with oil.