I've read up as much as I can and by all accounts other than a leaky system it most likely appears to be a faulty and Expansion Vessel, which may either require replacing or simply topping up with air, I've checked out the cost of an OM replacement unit, which seems to vary from around £52 to £130.
I have the instruction and maintenance books which came with the boiler, the expansion vessel appears to be relatively easy to check and the pressure reading should read between 0.75 and 0.9 Bar. I haven't got a Bar meter, but do possess a reasonably good quality tire pressure gauge, which according to a conversion website says that 1Bar is equal 14.5038 PSI.
In the instructions on replacing the vessel it says to 'drain the boiler as previously described', however I've searched the index and servicing instructions in the Manual from cover to cover but can't seem to find any mention of it, hopefully this doesn't mean that the whole radiator system has to be drained simply to undo and redo a single connector? also will the new vessel have to be pressurised with air after it has been fitted?
Here's a link to the brief instructions on the expansion vessel replacement, TIA for any help in advance.