Citroen DS3

[Taylorraz]

Hi everyone, hope this is the right place to post, we are looking for advice, we have recently had a Petrol DS3 64 plate we’ve had it for around 4 months but over the last 3 weeks when putting it into reverse there is a clunk the Revs go up but as soon as you dip the clutch it’s ok, as it’s losing drive (it does not kick out of gear) it only happens occasionally.
We purchased it from a Citroen dealer with the 12 months warranty( but they are asking for £96 up front for diagnostic)
Any ideas what it could be?

Thanks in advance
Mark