Don't restrict yourself to just the M series (unless you intend to build a collection), the A and autofocus lenses will also work fine on your cameras and you can take advantage of the extra features should you get hold of a later body.

Frenk,
I'm sure you'll be delighted with the 500mm - when it arrives!
You might also consider the Tamron Adaptall II 200mm F3.5.
I know you already have this focal length with your zoom but I have the 135mm F2.5 version of this lens and it is fabulous.
I'm thinking that with a 2x converter, you'll have a 400mm lens with a very usable max. aperture of F7.
You may find the 500mm mirror lens sometimes to be too cumbersome and inflexible and the 200mm/2x converter combination may suit your needs better.
You should be able to pick up a nice 200 for well under 50GBP (although I've seen them higher) and you'll be able to use the mount from your 500.

Make sure you choose the later Adaptall II version, as the older ones are often advertised and you may get the wrong one - although they may be optically identical.

With regard to the converter, prime lenses and converters are relatively easy to make and usually deliver decent results - I once owned a Vivitar 4 element converter, it was very cheap and actually pretty good!
For optimal quality (usually 7 elements) however, I'd recommend Pentax or Tamron.
Tamron's F-system converters are high quality and will enable you to use any Pentax fit lens or you could opt for their SP Flat-Field converter (8 elements I think) which is arguably the best 2x converter available but can only be used with Tamron adaptall lenses.

http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&category=707&item=3878583010&rd=1


Whatever you decide, I hope your 500mm arrives soon - it's a great lens!

To think we possibly brushed past one another John and didn't realize it - if so, you didn't miss much!

I agree that Pentax really should be there but I must admit that I'm finding less and less each year that really interests me. It's all high-tech digital wizardry, very impressive and too costly for the likes of me but you do get a sense of what the future has in store.

Focus on Imaging is on at the NEC at the moment (until Wednesday).
Naff all there Pentax-wise though and not a great deal of anything 35mm

Compare models and brands here: http://www.imaging-resource.com/IMCOMP/COMPS01.HTM

Well, most of the Sigmas I've handled felt loose and sloppy and I put it down to demo wear and tear.........perhaps I was being generous!

SMC is renowned but Tamron's BBAR is excellent too, I don't think I'd pay the extra for SMC and a Pentax badge. Also, you'd be stuck with a fixed mount on the Pentax variant and not have the advantage of the adaptall system (a selling point virtually unique to Tamron).

Sigma are ok but I've never been very impressed by the build quality. Most I've handled have been demo models and have probably had a lot of heavy use, so new ones are likely to be better.
Image wise, I think most people are happy with Sigmas until they try something better - cameras are often packaged with Sigmas from retailers such as Jessops and appear to be good value.
I believe that some Sigmas are very good but most are budget priced and that is reflected in the overall quality.
Regardless of make, most prime and moderate zoom lenses will give decent results but you shouldn't expect too much from a 'super-zoom', there are just too many optical compromises!

Interestingly, the Tamron 28-200 is pretty good and was actually marketed by Pentax (at a much higher price).

Well, I simply can't fault Tamron - especially the SP range!
With a PKA mount, you'll have the same functionality on an ist camera as you would with a Pentax A series lens.

For me, if you can't find or afford a Pentax, then go for a Tamron.

Takumar (Pentax) lenses are great but don't forget you need K-mount versions!
You can't go wrong with Tamron (my favourite independents) and some Sigma, Tokina and Vivitar lenses are also good.

I'd steer clear of Prinzflex and Sirius and any makes I've not heard of.

It's not fair to belittle any lens that gives a decent image, the focal length range that you require and comes at the right price but you should do a bit of research before shelling out for something expensive.
Less well known brands could be optically the same as their better known counterparts but why take the risk - go for Pentax.

Dessicants are a good idea, wherever you store your equipment.
I keep any sachets of silica gel that come in packages to save buying any.
A particularly good source for freebies, is your local shoe shop - they throw loads away!