You could do a lot worse than the DA*300 and a good teleconverter if you need some extra reach.
But it depends how much you're getting the 50-500 for......

This lot are very good.
http://harrowtechnical.co.uk/

Yea the k20 is a bit big, even for my 6'4" frame. I was going to buy the KM originally because of it's size but it was a stupid price when it first came out and with the cashback offer at the time it meant I could pick up a K20 for less than £100 more( from Jessops!). And I'm very glad I did!
Ok, it isbig...... but it's beautiful!

No 2 for me.
I must get a reversing ring and have a go at this macro lark. I have a M50 1.7 I could press into service. (actually I have at least 3, don't ask....)
Anyone know how a M135 f3.5 would do on a reversing ring?

look

Sorry if someone else has already found this but it looks brand new to me.
Sadly the 1.4 converter is missing.....
and no mention of a pentax version of the tokina 11-16 f2.8

Save yourself the headache and just buy the Pentax 55-300. It is a brilliant lens and at it's current price it's a steal.
I had the tamron and the purple fringing drove me mad. Can't comment on the Sigma because I've never tried it and now that I've got the Pentax I see absolutely no need to even look at it.

I have no idea.... but there is a short article here that can give you some pointers. You have to register to read the articles but it's free, so fill your boots!

+1 on the MX.

SRS are very good and if you're flying into Heathrow they are "on the way" into town.
Tottenham Court Road in London is full of electronics and camera shops and is also a good bet. Most will be prepared to match other stores prices but may not offer unless you ask! Don't be afraid to haggle a bit and if they won't drop the price they may throw in a UV filter or cleaning pen to clinch the deal.
Spectrum link near Warren Street tube have always been helpful when I've been there but they sometimes behave a little strangely when it comes to asking the price. More than once they have replied with "what would you like to pay for that", which is an open invitation to strike a deal so go armed with competitive prices!

Don't panic! I suspect it's camera shake. The very blurred portions in the pictures are where the animal is moving so it looks like a fairly low shutter speed is the problem for most of those shots. A 300mm on a DSLR is a very long lens and takes a bit of practice to hold it steady. I went through the same thing when I first got my 55-300. About one in five of my shots was sharp! I've improved quite a bit since then (but there's still a long way to go!). Take some test shots off a tripod and I'm sure everyrthing will come good. When you are out in less than ideal light look for something to rest against to reduce the wobble factor.

And I agree with the others, use centre focus!