Thanks all, so far. I keep coming back to same conclusions. The Sigma 150-500mm or the Pentax 300mm DA* with 1.5x teleconverter. I know the price has gone just above what I said for the latter but the advantage would probably be better IQ at 300mm.
gtis - I already have the Kenko 1.5 Pz-AF but it doesn't have SHQ on it. However comparing it with that picture it appears identical with the same contacts. SO will it autofocus with the DA* and non-SDM as well? I suppose the best thing would be to go to SRS and try out the combos.
Thanks,
Dave

There are number of threads regarding these but I wondered if there is a consolidated summary of the various permutations out there anywhere?
So far I have gleaned that:-
Pentax 1.7TC converts some fast manual lenses to autofocus but is expensive and difficult to get hold of.
Kenko 1.5 PFZ allows autofocus on non-SDM lenses and is ok value wise.
Tamron 1.4 is pricey and difficult to find but does allow autofocus on SDM?
Sigma - best used with Sigma lenses?
I'm not sure about other Pentax teleconverters.
I'm still trying to figure the way of getting fairly sharp 400-450mm autofocus for under £800.

Well I moved from the Panny DMC FZ-7 to the K-x with the DAL 55-300mm for bird photography.
I can say the following:-
1. It's definitely an improvement in quality. I don't regret the change
2. The lens is pretty sharp but is anything as sharp as that Leica in bright light!
3. Images will definitely look better
4. DOesn't like my Kenko 1.5x Teleconverter from a an IQ point of view.
5. Reach is fine but will never be enough. You get 500mm you want 1000mm!
6. Shutter is noisy but generally doesn't scare birds away unless close enough to not need 300mm!
7. Lens is slow to autofocus and hunts a lot but you learn to get the best out of it.
8. You can't possibly get a better set up for £500. Yes there is better gear out there but not for that money.
9. Best thing see for yourself as all my pics are taken with that combo.
They may well have had some photoshopping to get the best out of them
but that's mainly because I shoot raw with all in camera stuff turned off, pretty much.
http://www.flickr.com/photos/dcweather/

......... and this onesays K-x and K-m which it can't as far as I can see.
http://www.rocksphoto.com/goods-488.html
Dave

I think you are right as they seem to be for the K10D, K20D etc
This looks great if you have one of those though as it is unaffected in outdoor light being RF:-
http://www.amazon.com/RainbowImaging-Wireless-Shutter-PowerShot-replaces/dp/B004...=sr_1_fkmr0_1?ie=UTF8&qid=1293765349&sr=8-1-fkmr0

I think it was in DPreview forum I read it but I suspect the poster was wrong!
Dave

I read somewhere on a forum that you can get an RF remote for ALL Pentax DSLR's. Surely this wouldn't be true of the K-x which is lacking an auxillary connection for a receiver or does it work via the hot shoe?

......... sorry, I've just checked the lens and it might have been on!
Dave

I should add that the Jay picture above was taken without the rear uv filter. No particular reason, I just thought why add more glass but I haven't done with and without comparisons so maybe on what has been said above I will put it back!
Dave

Thanks for the comments. I'll think about while I am waiting for the 25-350mm

I have the Tamron 500mm lens and it is supposed to be better than the Sigma mirror. But you are right, it is difficult and never really can compete with my DAL 55-300mm over similar distances. In good light and with great care, some help from Photoshop it can produce some good results. I have found with the matched 2x teleconverter you don't lose that much so you can get 1000mm=1500mm 35mm for less than £150 with hardly any weight! It's not great but acceptable and fun!

at 500mm:-


Jay in garden by dcweather, on Flickr
Dave