The competition results are now posted!

The slight over exposure won't be a problem for negative films. It would be a problem for transparency film.

The lowest cost option will be a front lens fisheye converter. This screws into the filter thread. Cheap and cheerful, but it works.

A Pentax Adapter K will allow the use of screw thread lenses on the bayonet bodies, so that's another option.

Finally, eBay, camera fairs and second hand stock in shops will perhaps yield used fisheye lenses. It's a question of searching.

There are no current full frame fisheye lenses in the Pentax range, although there is one for the APS-C DSLRs (10-17mm) and one for the tiny Q (3.2mm), should you decide on a digital route at any time.

There's no problem with any Pentax lenses regarding dust. All lenses will get a few dust spots inside with age, as David said.

Sadly, the forum software has no facility for merging threads, so each one will have to stand on its own. Still, it's probably been covered very well in both threads.

I agree Gwyn, it would be useful, but we can't insist and many don't want to or don't think to.

The converter is also on the latest version of the roadmap. I fear it won't be cheap, but I expect it will be good.

ChrisA and gartmore have summed it up, many thanks for that.

Well you guys have sussed me out, it must be a selfie....

The Pentax and Ricoh GR batteries are the same.