Andy Burns wrote:

You can get socket bits in 3/8" hex fitting.

Yes, sorry, I put that badly: Screwfix had nothing *in the size I needed*.
The bolts were 10x140mm and the heads a bit chunkier than 3/8". I was
expecting that they'd have a set with different size sockets, but no.

Still, my socket-and-ratchet tool did the job.

Bert