Thanks Mat, I hadn't heard of K series lenses (never needed to pick up new lenses until now) but I'll get right on to looking them up. I'd found that Sigma do a few four/thirds lenses with
this one being of particular interest though it's very expensive.
And I've just come across a fact that Olympus OM series lenses work with an adaptor - though I assume that I could get a variety of adaptors to fit a variety of lenses. An adaptot from Fleabay can be got for as little as a £15 and very fast prime lenses in the OM series for £80.
Thanks Matt