Cap is available new from ford, most likely to be the heater matrix, seriously it only takes a few drops of leak in here to smell the whole bloody car out.
Don't worry about the gauge reading, that's fine, they are about as truthful as gordon brown!!
The caps will never 100% seal, some do have a very slight hiss even when new but shouldn't in theory.
If it's not using coolant, and the fans come on as needed don't worry.
Make sure you have the fan and pump loom rewired though, absolutely essential for health of the engine.
tabetha