I'd go down the NEBOSH General or Construction Certificate route - much more likely to strengthen your CV. General could be much cheaper e.g. by distance learning or at local college which will get EU grant. Construction Cert more expensive but more likely to impress if you are looking for a job in construction. Without knowing your background not possible to say where three day CDM course would take you but my initial guess is that it would not get you through APS membership hoops, in which case probably minimal improvement to your CV (and not a lot of learning benefit as you probably know a lot of the content already)

You hit the nail on the head - no jobs about and people with far better qualifications (who are also struggling for work) Sorry, thats the harsh reality of the current jobs market. Keep trying, something will turn up.

New Member here I have read a few posts on here about people wishing to become CDM/Cs, but it's not really about just taking a training course, and hey presto your a CDM/C. The job is connected with health and safety, but it's a whole lot more, in order to do the job successfully you really need quite a lot of design experience, plus a lot of knowledge of how the design process works, sure there are a few who have not got this experience (I really don't know how they demonstrate competence) I have met them. Read the ACOP, it tells you exactly what the HSE expects, and in these tougher times many employers are asking for exactly that.