Which replacement water pump?

Rod10

The inboard water pump in my 2005 Bessacarr Cameo is playing up, sometimes working and sometimes not. It's not micro-switch related because it stops working whether using the shower, wash-basin or kitchen sink. This leads me to think the pump is failing.

The pump is a Posiflow 3000-151-2201 but I can't find a direct replacement and wonder what alternative pump I ought to buy?

Also, is it a fairly easy DIY change to swap pumps?

Thanks in anticipation.