Ok, heres what I know....
It originally came with a Pentium 75Mhz (wow!).
Next to the Pentium (to the right) are three jumper connectors, one for 50Mhz, 75Mhz and 90Mhz.
In the centre of the board are also two more jumper connectors which do affect the Mhz, but so far, no matter what combination I try, I just cannot seem to get it to read more than 90Mhz.
I spoke to a guy at a computer fair on Sunday and he told me he would be surprised that it wouldn't jumper more than 90Mhz, he reckoned the difference wouldn't fry the board.
He said if that failed (and another vendor said it also) that an Overdrive processor 'should' do the trick, although they are hard to come by (I have just bought one from Ebay for £10, 120Mhz).
So with that information, does this ring any bells for anyone?