So... having explored many of the options, I've settled on using 6 of the Gen7 EndStops (v1.2) by Traumflug: http://reprap.org/wiki/Gen7_Endstop_1.2
I started out looking for kits, and other lazy/cheap ways of obtaining equivalent parts, but can't find anything with reasonable shipping costs (there is quite a bit available in mainland Europe, but the shipping is more than the parts cost!).
If I go it alone, getting the parts from Farnell will cost £12.73 + shipping, but Farnell have a minimum order of £20 - so would have to order the rest of the electronics at the same time to make it economical.
Before I commit to that, I've emailed Traumflug to see if he has any kits available at a reasonable price - fingers crossed.