Anyone know what pressure (PSI) the air con low side should read on a fully filled system ? mine reads 25 after a regas that I think may be a little low.

Refrigerant pressures in any compressor refrigeration/AC system are generally not very useful in determining how full or empty a system is.There are way too many variables - not the least of which is the ambient temp. As you are in the UK and not in balmy Oz, and it is winter up here your pressures both high and low will be lower (there should be a temp scale on the outside of the pressure scale on your gage too). That said, 25psi at idle does not sound off even in summer if you are cooling very well (we are talking about a running system? Pressures on a stationary system are next to meaningless).

All of this is why the refrigerant is filled by weight and not pressure(s). If you are unsure how much refrigerant has been added, the best way to determine if you are OK is with a thermometer on the suction line - or simply looking at the frost on the line. After running the AC for a while, you should see frost on a good section of the suction line. This indicates a saturation of the evaporator with liquid refrigerant in the process of evaporating (= enough refrigerant). The frost should end before the compressor, though, otherwise the compressor may end up sucking in liquid refrigerant and destroy itself (= too much refrigerant).

The frost line, BTW, is a good method for all who are tempted by the silly spring 'Come in and have your AC checked' ads to check the condition of the AC themselves. 90% of the time these checks are utter baloney and a waste of everyone's time - except for the sales guy who will try to sell you some other service or part... 
RRC 2Dr, RRC 4Dr,
P38, and 2 L322s
(wife thinks I'm nuts - prob right, too)

Very good reply thanks, so the frost line is the low side line from heat exchange unit to the compressor, the same line that I'm taking the pressure reading from ?
I was expecting to see circa 30 to 35 PSi with the ambient being 9 deg C.