I bought a new battery from Amazon. It is a Yuasa Ybx7005, it fits perfectly, as it is MADE IN JAPAN. I thought that alone made it worth the 119.95 it cost.
It is part of a new range of aftermarket Stop-Start batteries they have introduced, perhaps the demand is not sufficient to justify UK or EU production, or they haven't rolled it out yet.
Its got a translucent case so you can see the levels easily, and filler caps so you can top it up.
Am I likely to find an undamaged "insulator, battery" part no. 2889942020, as it has gone missing, probably discarded when the battery was changed in the past.