I have one of these filters but the nozzle for the outlet pipe from the head unit is slightly warped out of shape and not perfectly round. Therefore it leaks slightly. Any ideas how this could be fixed. Would it be possible to remould it somehow?

Try taking off the locking ring and wrap the hose with PVC tape.

If you wrap it tightly by stretching each wrap around you will achieve a lot of pressure and the hose should mould to the shape of the pipe.

if it takes 12.5mm tubing, you can get it from redwood aqautics, about $8 a metre from memory (we had to replace ours about 6 months ago) very good quality tubing

or you can get the same stuff from bunnings or mitre10 which is a lot cheaper, but it does kink and bend a lot easier and not in the same league as the stuff from redwoods