The lamps were rated at 18W. I intended to change these to a lower output as I wanted background light from these undimmable lights. I bought some 10W bulbs only to find that they would not fit. All I can do is to buy 18W warm white bulbs that will give a more restrained light output.
After some research I now know that there are several different and incompatible bases. For electronic control gear there are three called G24q-1(under 18W), G24q-2 (18W) and G24q-3 (over 18W). It seems that the voltages are different. There are similarly three for conventional control gear called G24d-1 etc.
No doubt many of you know all this already. But if like me you are new to these fittings, be warned. You must choose the correct wattage in the first place because you can't change later. I shall be buying conventional fittings from now on. Unless of course someone knows better than me?
There is a good data sheet from Osram which sums all this stuff up. Download it from CPC at http://www.farnell.com/datasheets/44969.pdf