I have had several customers with similar issues and currently have 3.
They are all salamander pumps.
Worked fine for several years and then pack up due to age (normal wear and tear).
A replacement (like for like as much as possible) is fitted by themselves/a friend/a plumber as it's a simple job. (note the word "fitted" and not "installed").
Then the problems begin and they call Salamander under warranty!
I've seen, first hand, the installation criteria checklist that the "approved" engineer has to complete and have been employed to put things right according to the criteria.
It is a very extensive checklist and I have never seen an installation that would completely satisfy it. Yet a salamander pump worked happily for years under these installation conditions!
So, what has changed?
Manufacturing quality and the warranties, that's what.
I've had several customers spend hundreds of pounds meeting the installation criteria, only for the warranty replacement pump to pack up, within a year, and for salamander to offer to send out another "approved engineer" at a cost... Or just to send out a new pump (free of charge) for someone else to fit.
Oh, and I had to send dozens of photos and a written description proving the installation had been "corrected" to meet their warranty criteria.
I'd say replace with a Stuart turner pump, but it's likely that they will soon follow salamander's direction to remain competitive with them!
Aqualisa are another company that have ridiculous "new" installation criteria!
As above... Put in an unvented cylinder if you have the mains flow and pressure.