Hi Guys and Gals...
A final update; it was indeed early signs of a head gasket failure.
After waiting a week or so for MJS Auto & Marine to visit me over Christmas, Mike made the 3 hour round trip from Littlehampton to London. Mike is a very knowledgeable MG enthusiast and I would highly recommend his services (so thank you David for being the first to point me in his direction!). After making many checks and diagnostics Mike came to the conclusion there was signs to show the beginning of head gasket failure. This was deduced by using some form of pump on the coolant tank, when pressurized showed cloudy, grey smoke, showing there were some fluids mixing due to HGF.
Unfortunately, it was about 2pm and the winter evening was drawing in. Mike suggested he towed the vehicle to his garage, worked on it in the following week and that I picked it up from him at my convenience. Well... not particularly convenient if you ask me, however, I had little option. Mike would have returned to fix this issue but the following day his other mechanic in Littlehampton was due to leave and the replacement not due for a few weeks. Sure enough, I popped down to pick it up and all is now well - I am soo happy my little MG is finally back with me.
A scenic, traffic-free drive + A nice area to visit and spend the day if you're waiting for a service or pickup + A great guy and competitive prices + A garage full of MGs (about 4 TFs were there at the same time) showing real knowledge for this vehicle = a highly recommended MG mechanic.
It is a shame the head gasket is being replaced as I believe it was only recently skimmed 2 years ago (less than 10k miles!) but hopefully time will show that it makes a difference getting a good job done than a cheap, quick fix. Fingers crossed that's my MG fixed for the forseeable
Thanks again for your help and comments to those who contributed - you truly did help get me back on the road!