Hello Paul,
I know you have already received quite a number of replies with advice, so I hadn't taken a close interest in the thread but I knew there was something niggling me it until it hit me a few moments ago.
The pipework demonstrated in your pictures does not look like the standard of work most caravan manufacturers would achieve, it looked too neat! and the fittings did not look like typical items for the time.
You tell us the caravan is a 2003 model, but your picture shows the side of a Cascade 2 water heater. That does not make sense because the Cascade 2 water heater was taken out of production in 2000.
Carver sold its leisure product range to Truma in 1999, and Truma then closed down the Carver heater production, replacing them with Truma models in 2000 early 2001. It's unlikely any of the main caravan manufacturers would have designed any 2003 model caravan to accommodate a Cascade 2, they would almost certainly have designed it for the Trumastore equivalent, which is an entirely different shape and design.
This means either you have not got a 2003 model caravan it could be earlier, or that a previous owner installed an old stock Carver Cascade 2 in your caravan, which seems to be the more likely scenario.
This doesn't help with your immediate problem , but it might explain why we have not been able to give an instant solution. What I can state is that you should definitely not require a hammer to make the system operate properly, there should be a less destructive method of safe operation for draining for storage and resealing for usage.