Advice please (for some reason search result gives "forum error"!)
As expected the bolt holding the battery retaining clamp won't unscrew, I can just feel the nut on the end of the under the battery try turning.
It looks as though there is enough room to get the angle grinder positioned to grind off the head of the bolt and remove the retainer clamp, I'll then remove the battery tray to see how the retainer bolt nut was originally held in place
I've read threads where this problem has been encountered but, sorry, I can't remember the solution to the problem
Any help/advice will be most appreciated

Battery to be purchased: 096 Varta Silver, 77Ah, CCA 780A, £87.50 incl delivery from Battery Megastore

Retaining bolt fits into a threaded sleeve moulded into the plastic. When the bolt rusts in the sleeve turns with the bolt !
I managed to slide the retainer away from the battery enough to lift the battery out then used a long bolt fitted from the underside with a captive nut then the plastic wedge and a nut on top.

Andy, thanks for quick reply.
Did you use an angle grinder or drill to remove head of retaining bolt?

We used a long bar to prize the old one out and the new one back in. Good luck

Once you've cut off the head of the bolt, the battery can be removed, allowing access to the battery try retaining bolts. The clamp bolt screws into a brass insert in the tray, but the bolt rusts up, causing the insert to spin. With the tray removed, I knock the insert out, and glue in a wood threading insert instead. This then allows it to look original, and won't rust up if a stainless steel bolt is used.

I just encountered this issue and have used a wing nut, anti-vibration washer and a minimal dab of blue thread lock over and above @Nodge68 approach. Hopefully, should anyone other than me have need to remove or change the battery, there will be less likely to have someone use a "gun" to tighten (what would have been) the nut.