I've done the usual trawling through Ebay but still cannot find anything that falls into a reasonable range. Has anyone got any tips or tricks for overcoming this ?? I've looked at hides but most of the photography is to be done on common land so poses the obvious threat of removal by our light fingered friends. What are peoples thoughts on teleconvertors ??

Two ideas come to mind immediately. Firstly, super-long lenses are not strictly needed even by "big boys", but, if one is really needed, and cash is short, try a good mirror 500mm, or, if you're feeling adventurous, one of those pre-set long-focus lenses of the same focal length. The mirror has the advantage of focussing much closer (usually around 3 meters or so) and faster. Before the flames begin, yes, there are those pesky "doughnuts" in the image if it contains brightly highlighted bits in the background. However, in most cases, this is not a real handicap when used in a forested area. The pre-set has the advantage of a real, honest-to-goodness aperture as well as generally being contrastier than a mirror. I use both types as well as 100-300mm and 85-210mm zooms, which leads to the next thought:
Instead of a blind, use one of those netting types of hides. Folded, they take up little space and can be used anywhere. The trick is to find a spot, preferably next to a tree, set up your tripod and cameras and throw the netting over yourself and the set-up, leaving the area in front of your lens/lenses clear. Sitting is suggested as this will break up your profile so you don't look like a human. Best place is near a small pool of water with forest around it. The birds and other animals, naturally, will, sooner or later, come down for a drink. I don't know the situation in your neck of the woods, but around here there are a lot of snakes, some quite dangerous (cane-brake rattlers, coral, and copperheads are a few) that tend to hang around water a lot. The best advice I can give is to simply let it slide on by without undue commotion.
As to shorter zooms like a 100-300mm, these are most useful for accessing areas higher or lower than yourself or hanging over water or bog. Also, many times, even when just standing still out in the open, animals can be contrary and land or walk almost right on top of you. The zooms come in handy for such tight quarters. I once had an osprey (fish-hawk to some) land on a snag less than ten feet in front of me. Unfortunately, I had next to no time to get over the shock before it realized that there was a human presence and it took off (with its back to me, naturally). I learned to be more aware of such opportunities from that day and prepare for it as a matter of course.

from Deepinaharta, Georgia