Yes Sheila, that had crossed my mind as a last resort! It's far from ideal, it still relies on Milly being able to use the flap too so I still need to sort that out first, and because it will mean that his original microchip details will get smothered by the new ones, so it will no longer be able to be used to monitor his temperature...and while I'm not planning on taking them anywhere, apparently it can cause problems with pet passports if an animal has more than one chip. It's definitely something I would consider if everything else can be made to work, though. My usual vet is away until the end of the month and it's something I'd rather discuss with her than with any of the other vets at the practice, as she knows me and my animals, but it's definitely there on the 'things to consider' list.
I haven't registered it online yet, Sue - I must do that actually! I did buy it directly from them though so they will already have a record of its purchase. I have gone back to them this pm and made clear that there are 2 cats and 2 issues, so I'll see what they say tomorrow. I also asked whether it's worth considering trying the cat flap rather than the pet door (if Geoffrey can get through it...need to try him out with a hole in a piece of cardboard!) - don't know whether the technology/range would differ with the smaller flap and whether it would be easier for Milly to operate. Again, not ideal, but perhaps something to keep on the 'things to consider' pile. I'm determined to make this work somehow!