When you receive your explosives certificate (COER 3) from the Police and your Registered Store Certificate from the relevant body, you must then apply for your RCA document. This is done by sending "copies" of the originals of both your Explosives certificate and your Registered Store certificate to the Health and Safety Executive (HSE).

A letter explaining that you are applying for a Recipient Competent Authority Transfer Document for:-
Rocket Motors UN0349 and UN0351
Black powder UN0027 and U0028

The RCA is issued free of charge and is valid alongside your explosives certificate. You will also find a lot of other UN numbers on the RCA, but these are applicable to other items you will need an RCA for during your rocketry activities.

The address for the HSE is:-

Health and Safety Executive,
SPD Rose Court,
2 Southwark Bridge,
London,
SE1 9HS

Tel 0207 717 6205
Fax 0207 717 6690

I faxed my forms to them and got my RCA the next working day :-)