"Stuart Bell" wrote
| I recently moved into a flat that, as well as having 'standard'
| modern plug sockets, also has some older ones, with holes for
| three round pins.

Are these *old* sockets, or new sockets with round pins? In upmarket
properties (or those rewired by people with cluefulness) round pin sockets
are sometimes used for table lights controlled by wall switches.

Or someone might have (re)used the old wiring for speakers, etc.

| I have no idea whether these are live, and guess that I could use
| one of those mains tester plugs in order to check whether they're
| wired up correctly, but:

| a) Are they potentially usable?

Potentially yes, but if they are old wiring there is probably good reason
for not using them and disconnecting them (ie the wiring is knacked, still
on old fuses, etc). Knacked wiring will not necessarily be revealed by a
mains tester plug; more extensive inspection/test would be required.

| b) Are there adapter plugs available to plug in equipment with new-style
| plugs ... or replacement plugs to connect to the equipment, perhaps?

2A and 5A plugs are available in B&Q. 15A plugs available from other places.
Especially if they are 15A plugs you should use a 15A
plug-to-extension-socket, so the appliance retains its 13A *fused* plug, so
that an appliance that requires a 3A fuse is suitably protected (assuming
you have the correct fuses in your 13A plugs to start with) rather than
relying on a 15A rewirable fuse in the fusebox.

Owain