Okay then... What I think I'll do this weekend - weather permitting of course since it's another wet 'n dull day in the UK... Is do a partial water change. I'll pump out about 50% of the water and then fish out as much debris from the bottom of the pond as I can. The filter will be okay since it always has a level of water in it. I'll then top up with conditioned water and transfer my plants to the pond and take it from there. I've got some hozelock pond plant food so I'll feed them on a regular basis and monitor the condition of the water closely.

If its sunny I may also keep a note of how much sun the pond gets.

Thanks for all the help.