Well 2nd one in to welcome you.
I've heard Andy there will soon be doing his lap in an affordable auto and missing his beloved Reliant Robin whilst doing it I'm sure.
On to the matter at hand. A helping hand with your hand means the top is not quite working correctly. Though not as bad as a Spyder Veloce I once owned that required two sets of hands to stow and sometimes three to deploy
but I digress... IF you have any warranty, which I'm hoping you do, get a dealer to have a look. The top is a pretty complex affair and its likely after 2-3 years of use it just needs an adjustment hopefully not more than that. Check out the search option for top issues they do happen as can be seen by some posts. Check with the original owner if any work on the top has been done in the past too.
Other than that I'm glad to see you chose the best colour the Z comes in
Hopefully it will be sorted soon and you'll be zipping around ol' Blighty in no time with the top down.