I don't have the plate to hand, but from memory I think it's a pretty standard procedure at Cardiff, in which case:
First of all, check your clearance. The controller may clear you for the lateral part of the procedure, but want you to remain at 3000', in which case he'll use a phrase like "Cleared outbound for the procedure, remain at 3000 feet."
Assuming that's not the case and you are "cleared for the procedure", you may begin your descent as soon as you pass overhead the beacon and start the turn. This is because the beacon is the IAF, so as soon as you pass overhead it you have begun the procedure.
I don't think you'll find anything written down about this specific case (of extending the outbound part of the hold), because it is no different to the general case that you can descend once you have passed the IAF and established yourself on the outbound leg.
FFF
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