So, are you absolutely certain that the new pump is pumping in the same direction as the old one, ie, the arrow on the old pump was also pointing downwards??, Is you system as I showed above or can you post a photo of it?, its strange that you are getting problems at CP2 even though it is a bit high so CP1 may sort out the problem, was that system chemically cleaned or anything like that between pump changes?.
This is the old pump and if it appears you will see what I mean by upside down. I don't have any other image so can only presume the British gas engineer who put the new one in ensured it is pumping on same direction. No chemical cleaning I am aware of.