I then insulated the electrical connections of the pump using silicon and connected the pump cable directly into the cable of the main unit inside the bottom of the egg thus protecting the new pump with the RCD. I did not disconnect the old pump electrically as my pump still operates but not well.
I then covered the new external pump with a plastic box to stop anyone touching it and then fired up the system and bled the pipes with the airscrew on top of the egg and on the new pump (recovering it afterwards)
The new pump works fine, according to the specs on the old pump it needs to shift about 500 gallons an hours so a central heating pump will shift the metric equivalent without any issues, my pump moves up to 3 cubic metres of water per hour and this is fine, any more you might not heat the water that well, any less and it wont filter properly.
Electrically this is probably not legally safe, for me it works fine and I dont have any issues with the electrical safety, it works and I have no rust problems from using a metal pump, just make sure the water is drained and not left standing over the months when you don't use the system.
I have used mine all summer without any issues and if anything it filters now much better with this pump.
I take no responsibility for any changes you make to your system, this is not advice but merely a solution that I came up with to fix my unit, how you fix yours is entirely up to you , it works for me, If you undertake any changes to your system you must be competent with electrics and plumbing.