After a little research I think I've found the right place to register my boat:
Registering your pleasure craft : Directgov - Travel and transport
In summary then is the following correct to use my boat in Spain?
RYA Level 2
RYA Coastal Endorsement
ICC
Insurance (already organised)
I must register my boat
National flag on stern
Life jackets for everybody on board (I do this anyway)
3 Flares
Signaling Mirror
Fog Horn
First Aid Kit
Torch
Anchor
2 x Oars
Pump and Inflatable Repair Kit
Bilge Pump
Boathook
2 Mooring Ropes
On another note, although with hindsight it's not very secure, but we just pull the boat up onto the Beach and leave it there overnight. Plenty of others do the same and we've spoken to the locals and they assured us that everybody does it. Fingers crossed somebody doesn't decide to pinch it this year. ;-)