Barry was right...
The rest of the removal wasn't easy as such but the connectors were the sticking point.
Rewinding a bit, I decided on removing the filling hose to the tank due to it being the most viable means of getting the siphon in there. After wrestling with a 17 year old jubilee clip, I managed to get this (and the even more stubborn breather) hose off and drained £30 worth of Tesco's finest out of the tank with a Draper squidgy bulb type siphon which worked very well.
Next up was the four tank-holding bolts. These were a pain for one (an awful design), but an extra pair of hands eventually got them free.
Finally, we got to the pump fixings themselves, and after 20 mins of gentle coaxing, it was obvious that they were not going to come quietly. Eventually, they were brute-forced off as per the first photo in this post. I'll have to find replacements!
Anyway, tank off and it's not in bad condition;
Also not bad is the metalwork behind the tank which looks very clean;
Having said that, the rest of the rear structure is looking a bit crispy, so I cavity-waxed the out-riggers while I was there. I need some more Bilt Hamber stuff though as the can wore out before I finished.
So.... broke stuff - removed stuff sort of day.