Thanks tail, what you said about other species tolerating them is really interesting and exactly fits with what I observed, which is that I found this ant walking around in an area infested with Formica rufa, and I wondered how they survived.
My book ('Ants' by Gary J Skinner and Geoffrey W Allen) lists five Leptothorax species for the British Isles, namely L. acervorum, L. nylanderi, L. interruptus, L. tuberum and L. unifasciatus; of these, L. unifasciatus is restricted to the Channel Islands, so I assume it's not this, L. nylanderi doesn't have the dark tips to the antennae that you can see on the photo, L. tuberum has short straight spines on the propodeum whereas L. interruptus has long curved spines - if you look on the first photo you can see these quite clearly just above where the rear legs join the body - these look long and curved to me, making her L. interruptus rather that L. tuberum. As for L. acervorum, apparently it has 11 segments on the antennae whereas the others have 12; it's a bit hard to count them when she's alive (even with a microscope) and I don't want to kill her because I'm nice

, so I can't definitely say it's not L. acervorum, but anyway the spines on the propodeum make me lean towards L. interruptus.
About the photos, all I've got is a normal 'point and click' digital camera (Kodak 'EasyShare' Z650, 6.1 mega-pixels), and basically I just put it in the 'close up' setting, turn off the flash, don't use zoom, get as much light on the subject as possible, and give the ant some honey so the damn thing will stand still for a minute! Then get the lens as close as I can before the camera stops auto-focusing, and take the photo. Also have the image set up as big and high quality as the camera will allow.
Here's an offtopic (sry) picture of a Formica rufa caught in the same place as the Leptothorax, using the same 'technique':