I have gone for the Panasonic battery.
MX5parts are out of stock but I found one at my local Mazda main dealer, which is only 2 miles away. It was £22 more, but I saved on the £10 delivery fee so I’m happy with that. The guy at the dealership (who I know quite well as they look after my Mazda6 Sport) told me that Mazda use Panasonics in almost all the cars they sell. That’s good enough for me, and I’m not going to quibble over a few quid.
I’m currently waiting for a vent tube and battery tray, both of which were missing. In the meantime I’ve treated the surface rust in the battery well and it’s all hunky dory in there now.
Thanks again to all for the advice. 