The convertible hardtop has just started sticking in various positions. The system was repaired at a dealer about 18 months ago. Any ideas?

Since you've provided so much detailed information this should be a cinch . Seriously though, see if you can determine if the pump is running when the roof stops moving. If the pump isn't running it may have overheated and tripped the thermal cut-out. Just a WAG. Checking for fault codes is your next step.