Have to replace the kitchen strip light but im having trouble finding the size.
its probably very old but all it says on it is osram-the make.
and L/58 w/ 23.
all the ones ive looked at in b&q are in exact feet.
anyone know of a place that does loads of sizes.

Try Osram's site, a quick search produced this:

Tubes (http://www.osram.com/products/general/fluorescent/2.html)

Just measure it.

They come in 2ft 3ft 4ft 5ft 6ft etc etc, also make sure you get the right diameter one.

Other than the DIY stores try

City ElectricaL Factors
Edmundsons Electrical

I have measured it and its 58 and a half inches from prong to prong.
ive detatched the prong bit to get the right diametre size.
looked on sites but no one has this exact length.
might try wicks.

Have you tried Maplins?