can anyone explain to me why

http://www.jessops.com/online.store/categories/products/Sigma/70-300mm%20f4-5.6%20APO%20Macro%20DG%20(Pentax%20AF)-26772/Show.html

is so much more expensive than

Sigma 70-300mm f/4-5.6 DG Macro (Pentax AF) - Jessops

The first one has the APO which gives better / more low dispersion glass to reduce chromatic abberation.

Apart from that, they're pretty much the same lens with the same focal distance / aperture ratings and both provide 1:2 macro.

The first one will probably give better quality pictures wide open and/or on the edges of focal distance than the one without the APO lenses.

The difference in price in Jessops is more than the normal difference between the lenses. Sigma's RRP is under £150 and £200 for the two models, but you're more likely to find the non-APO one reduced as it's probably an older lens whch is effectively being replaced by the APO version.

thx - so is it worth the difference, or if I'm after a cheap way of getting a lens that does 300mm and macro, should i just go for the non-APO one ?

If you're after macro, I wouldn't worry too much. The optical difference in them will be noticeable under certain conditions, but the difference in price, I'd go for the cheaper non-APO version.