Wills, the problem was not the 22m elbow that replaced the FX5 double output, I had created too much tension on the outpipe (trying to ensure no sagging that would lower power output) and it was the standard rubber junction that gave way because I was clamping the lower end of the rubber connection to the G-clamp.
After today's drama, I have now clamped the upper end of the connector and slightly lowered the tension in the outpipe.
For <£2 per FX5, the plumbing 22m elbow joint mod is really worth doing in a tank where fish enjoy current. I might try and get a video clip to show you what a big difference it makes over that standard double output nozzle.
Heater is fine, I was aware in theory that the Renas had an auto-cutoff. I'm not sure if the same applies to the standard Juwel heaters that come with the tanks?