Okay so quick update on this. Had a look at the car today and after filling the car with water and bleeding the system she's still building pressure so the most likely causes are the coolant bottle cap (replaced) thermostat (replaced) or the headgasket gone at the fire rings pushing exhaust gasses into the coolant. I will be getting a sniff test done tomorrow to confirm this, I just hope the head hasn't warped so I don't have to get it skimmed or would it be advisable to skim the head anyway?

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I would only get the head skimmed if it is warped or damaged.

think its the liner heights that is one of the most important things and can give trouble if not sorted.

I don't think there is alot of scope for skimming the head as the limits are so small. Hopefully someone can confirm if this is true or not......

First of all, read this if you're doing it yourself:
Head Gasket Guide - K Series

Second, was it a genuine cap that was fitted? We've seen a lot of bother with yellow aftermarket caps.

Finally, if your liner heights are spot on (and assuming it's a 4 pot K Series), give the N Series Head Gasket Kit consideration:
N Series Head Gasket Kit

It contains 3 of the parts NAC MG developed to 'fix' the K Series.

Yeah I got the cap off yourself Matty when I was ordering the thermostat. I won't be doing it myself as I haven't got the time nor the equipment to do it and I need it done asap. I will be giving it to a former MG-Rover dealer in Dublin who still do parts and servicing

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Finally got some good news on this. The mechanic I initially brought the car to diagnosed HGF through a sniff test however as he was making excuses to not do the HG I took the car to an old MG-R dealer and they done a sniff test and compression test and it confirmed the HG was grand. The thermostat I put in before Christmas was a dud so they replaced it with a genuine one that I got off DMGRS. The car will be back tomorrow. The only problem is now that the fan at the back of the rad isn't kicking in, the one at the front of the rad is grand and the car is okay to drive out on the road I just need to watch out in heavy traffic. Pretty happy with it so now I just need to know what's involved in changing the fan on a 4pot K-series. How hard of a job is it?/

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Good news.

Do you mean the fan in the engine bay? I'Ve not seen one there before but the one between the bumper requires the bumper off.

glad it was sorted and not as bad as it could have been.

Yeah I was delighted it wasn't the dreaded and it just goes to show how valuable real experts in a certain field can be. It is the fan in the engine bay. The one between the bumper is working fine but the one in the engine bay won't start unless you give it a nudge to get it going