My undertanding is that both of these are double pole switches, the
difference being that the FCU has a fuse protecting the live wire on
the spur. Is this correct?

Yes.

If so, which should I use?

You may use either, provided they run to a single accessory. If you run to
multiple accessories, you must use the FCU.

Finally, I assume that both of these switches have 6 terminals - L,N,E
for the mains and L,N,E for the load. I then wire both ring cables
into the mains part and the cable for the spur into the load part. Is
this correct?

Yes, although the earth terminals might be shared, or not marked for load
and supply, in which case you wire to the most convenient earth terminal. I
used 20A DP grid switches, where you can get 1-2 DP switches in a single
box, or 3-4 in a double box. It gave a neater appearance than having 3
adjacent FCUs.

Christian.