Instead of a 3-bath kit, I suggest you try to find a 6-bath kit. Kodak sells a 5-liter size kit, or you can buy separate components from Kodak or Fuji, but they're sold in much larger quantities only, as far as I can tell. I don't know about availability in the UK, though. In the long run, the Kodak kit will be less expensive (at least in the US), and you're likely to get better results from it -- there are compromises required in squeezing six steps into three. The drawback, of course, is that the 6-bath kit is more of a hassle to use, with more chemicals to mix and more steps when doing the processing.
OTOH, if you've just got a few sheets of expired film and you're not sure if you'll be doing more E-6, then starting with a 3-bath kit might make sense. Just remember that if you get unsatisfactory results, it's likely because of the expired nature of the film or the shortcuts in the 3-bath kit.
FWIW, I've done E-6 using both 3-bath and 6-bath kits. I don't shoot a lot of slide film, and my processing is done via manual tanks, but I get results I consider acceptable, although I get more consistent and slightly better results from the Kodak 6-bath kit.