Thank you Theolegit for the links
Today I have placed an order for the square leccy socket from westfaliasparesuk.com
I've had a look about and I'm pretty sure I can modify it to take the water filler.
Expensive though for a bit of plastic

But at the moment the rain will be getting in behind the panel so thought i'd sort before winter set in.
Twist my nipple nuts and send me to Alaska.