Less, but trying to improve the quality of those that I do take by taking more time to compose properly.

Always RAW for the same reason as snappychappy. Also you lose so much detail from a JPEG in the highlights and dark areas whereas in RAW the detail is there waiting for you to pull it out in PP.

The Sigma 150-500 OS HSM is currently on offer at SRS at an incredible £599 and is around the same weight as the 60-250 and gets rave reviews.

HOW BIG!!!!!

Please read this link
Go to the following chapter... 2.5) Be careful about autofocus
The K-5/Pentax is not the only camera/system that suffers in these conditions.

Barri, what do you have in mind?

Good point John, however it will cost as much/more than the DAL 55-300 and is rarer than rocking horse excrement.
I still think cropping from the DAL 55-300 would give better results than the Sigma 70-300 with any TC.

As David has said, putting a TC onto a zoom is not recommended particularly one that starts at f4 as a 2x TC will make it f8 so very dark images, slower shutter speeds and hence blurred images, not to mention going through the additional layers of glass which will also destroy the IQ. I have a Sigma 70-200 f2.8 and even that loses IQ when used on a TC.
Save the money you would spend on a TC, add it to a pot for a Pentax DAL 55-300. This lens is a stunner for the money and massively out performs the Sigma 70-300. It comes in 2 varieties, the DA and the DAL. the DA has a metal mount, quick shift focusing and a lens hood, but otherwise it is the same lens as the cheaper DAL both have the same glass and so the same IQ which is significantly better than the Sigma.

I think PF means purple fringing i.e. chromatic aberation, am I right Mike?

The only way you are going to get anything in light that low is to use a tripod and a very long exposure time