Prior to running an electric shower I was seeing 247 volts (it fluctuates
quite a lot up to 248V). With the shower on it dropped to 234V. This
seemed like quite a large drop, does anyone agree?

216v is the lower end of the limit so you`re well within the boundaries.

Are you fed by a rural network, or are you in an urban area with mainly
undergrounded cables ?

It could be that a fuse has blown in a substation feeding one end of the
cable, so in effect you are now fed radially (one way) rather than from
both ends.

Many overhead lines are radial by design, so if you are more rural you
may well be stuffed unless you get something a little more damning.

--
Please add "[newsgroup]" in the subject of any personal replies via email
--- My new email address has "ngspamtrap" & @btinternet.com in it ;-) ---