I need to replace one engine mount on my 1GM10. Ive just seen 4 on e-bay at £100+ Is that the price of these ? And the ones on e-bay are off a 50hp engine - are they interchangeable with a 9hp ? - Feeling out of my depth here!
You need the correct or compatible mounts. It's very unlikely that a set of engine mounts for a 50hp engine will even fit your engine and bearers, let alone that they will be the right elasticity. The 1GM10 uses quite springy mounts to allow it to dance around a bit without doing any harm. The correct Yanmar mounts are about £70 each I think. That may seem quite expensive but they should last something like a decade or more in normal use.
There's a wee problem just replacing the one. Sorry but they're matched for resilience and compressibility. As the mounts age, they deteriorate roughly in the same proportion so when you replace one, it upsets the balance. I went into this quite deeply when I replaced mine last season.
Of course you can keep the good old ones and in future you will be able to replace a broken one with a "used" one without upsetting the balance as much.