Erm!!!
Where can I get hold of one for 3157 a smokers satisfaction????

Assuming you are talking about a replacement certificate, the RYA site gives the contact, cost etc.  I found it a very easy process, even including a name change (For the boat, not me).

You obtain them from the RYA 

RYA House, Ensign Way, Hamble, Southampton, SO31 4YA, United Kingdom.
tel: 0845 345 0383 (charged at local rate) 

Or: 023 8060 4202 (Charged at National or local rate)


- here is the extract from the Class Rules

a) No boat is entitled to race unless the Owner holds a valid certificate in
his own name, issued by the Royal Yachting Association, with a current
buoyancy endorsement. No boat is entitled to race in the class unless
the Owner is a current member of the National Merlin-Rocket Owners’
Association.
(b) Other spars, sails, centreboards or other gear may be used and
alterations may be made to any part of the boat without invalidating the
certificate provided that:
(i) The boat continues to comply with the class rules, and
(ii) the boat is measured to the extent necessary to ensure
compliance with the class rules, and
(iii) the band measurements are unaltered, and
(iv) only sails which have been measured shall be used.
(c) The certificate becomes invalid and a new certificate shall be obtained
if:
(i) The boat ceases to comply with the class rules, or
(ii) the boat measurements are altered, or
(iii) the boat changes ownership.
(d) A certificate can be obtained in the following ways:
(i) For new or re-measured boats a measurement form completed
as appropriate and signed by an RYA Approved Merlin-Rocket
Class Full Measurer together with a current buoyancy
endorsement shall be forwarded to the Royal Yachting
Association together with the registration fee.
(ii) On change of ownership, by forwarding the old certificate with a
current buoyancy endorsement together with the registration fee
to the RYA.
(iii) For MERLIN class boats built before 1 February 1951, by
forwarding a valid MERLIN class certificate issued before 1 April
1951, with current buoyancy endorsement to the RYA.*
(iv) For ROCKET class boats built before 1 February 1951, by
forwarding a valid ROCKET class certificate issued before 1
April 1951, with current buoyancy endorsement to the RYA.*
(e) The RYA may at its unfettered discretion examine any boat, refuse to
issue a certificate or cancel the validity of a certificate already issued if,
in the opinion of the Chairman and Secretary of the responsible
Technical Committee, there is any non-compliance with the
specifications or rules.