Vauxhall Corsa new battery fitted, alternator works fine but battery won't keep its charge, battery checked as ok so something is draining the power

You will need to check for a parasitic drain .

Also LOAD test alternator .

Does seem that somethings draining it if going flat overnight as above check for current drain

Thanks for the replies, any idea how or where to look for a drain? I guessed it may be a short somewhere but where to start? My daughter is going nuts as she can't get it repaired and being the dad I'm feeling it big time! ?

Here is an example of how to check ,

https://www.youtube.com/watch?v=KF1gijj03_0&feature=emb_logo

Thats great, thanks! I'll let u know how i get on