Assuming that the L5 Certificate provider also offers the Diploma, you might be best to speak to them in the 1st instance to see what they might be able to offer to upgrade to the extra modules (I think it is only a few if I remember rightly?).
Are you doing the Certificate rather than the Diploma due to employer funding?

Unfortunately it has become a trend recent with course providers only offering the Cerificate, and stating that students do not want to do the Diploma! Lets look at the credits that are needed for the L5 quaklifications. The Certificate is 32 credits and the Diploma is 44 credits pit this against a HND which is 240 which is a massive gap, yes the Diploma has more modules but these are hardly diificult to complete. People who have the Diploma will be a little bit more rounded qualification wise than those with the Certificate. If you are going to collge then watch out for them trying to run a course on reduced hours which does happen on the L5 Certicate courses. If your not happy with the how your course is being run then contact the Q&A department at the CIPD. You can upgrade a L5 Certficate to a L5 Diploma, easiest way is via distance learning, however it is more costly this way! If you can do the Diploma then do so!

Thanks for the replies and comments, I appreciate it.

The provider only offers the L5 certificate, and for my day-to-day career plans and progression that's all that I need at the moment. Longer term I may decide to skip the L5 diploma and go for an L7 qualification: I have a reasonable amount of practical experience and the CIPD qualification is to help strengthen my understanding of the theory.

The provider is a local college that we've used before and comes with some excellent reviews and feedback from colleagues. Exciting times ahead!

In reply to Garry:

You might find something in here useful about skipping a level, personally I wouldn't do it because you won't cover a lot of stuff at higher levels that you will need to know workforcedynamics.wordpress.com/.../

In reply to Paul:

Thanks for the link Paul, very useful and after 2 weeks off where I've been thinking about options it was very timely.