Which tele-convertor?

Unless you can find a good teleconverter for under £200 I would say to buy a DA 55-300mm (or the older DA 50-200mm) instead, you'll get much better results than you will with the TC and zoom.

You can currently find the 50-200mm for £100 or less brand new on ebay, I have never seen a teleconverter worth owning for less than that and the lens is probably a similar size and weight to a TC.

I wouldn't recommend using a TC with a wide to standard zoom, unless you intend to use it only at the 50mm setting.

Regards,

John

Agree with the other posters. Teleconverters and wide angle zooms aren't an obvious match. And by the time you pay enough money to get a decent teleconverter (some would claim that is an oxymoron) you would have put it to better use buying a telezoom to complement the lens you have. Best bang for minimal bucks is probably the Tamron 70-300. Cost effective, small, light, decent enough IQ and weather resistant to go with your K-7 would lead you to the 50-200WR. Best IQ for a reasonable price is the 55-300.