Connection of hot tub

I would be careful with using extension leads. First the cable is not protected against mechanical damage, secondly I have seen melted extension leads connected to those small lazy spas. RCD will not protect against overheating of the extension lead. If you are going to use extension lead make sure it is fully pulled out to prevent overheating and has thermal overload protection build in as well. The best solution I would suggest is to have another IP rated socket with SWA cable buried so the lazy spa can be connected directly (of course protected by RCD).

Shouldn't run extension lead. Better extend outside stock close to the hot tube.

I would strongly recommend not using an extension lead. Even though it may be RCD protected there is a chance that RCD may fail. Meaning if a fault did occur you would not be protected. My suggestion would be to install a dedicated supply to the hot tub and install an isolator local to the hot tub to allow protection of the appliance without risk to the client that you would have using an extension lead.