please help i was out today and my gas gas produced a cloud of steam and threw quite a bit of creamy oil from the crankcase breather im guessing its goosed any ideas please

It sounds like you got a blown seal, oil and water mixed together, maybe a water pump seal

if its presurising the gearbox to blow oil out it could be a crankseal, or as you say, the waterpump is filling the gearbox (likley)

Thanks guys i no this may sound daft but would the 280 barrel fit a 250 bottom end as i have a perfectly good one sitting in garage that the head went on it just a thought cheers

Chances are that the motor has over heated due to the failure of the thermostat or fan. Assuming it's an older pro with the in line stat, could be the stat will have blown out of the housing creating the cloud of steam. As said previously, sounds like the water pump seal has blown due to overheating.

Cooling fan failure will allow the engine to get hot but if you know it was working then; A leaking cyl'head seal will boil the water and presurise the water system.

The pressure should all escape via the Rad vent pipe if this is kinked/blocked then the next weak point is the pump seal.

Worn/damaged crank seal (right hand side) will create pressure in the gearbox and force the oil out of the breather.

Your spare cylinder should fit but the 280 head has a different size inner O-ring seal and is too big diameter for the 250 cyl.

ie, the head will go on but the seal will be wrong!