Quick tip for changing central heating type pumps....
If you look at your pump, it will normally have between 2-4 hex Allen type bolts, holding the head onto the body. The secret is to buy a pump that's the same make, as the one you are replacing.
Isolate the pump, both electrically & mechanically. The mechanical side is accomplished by shutting the valves above & below the pump. Remove the 2-4 Allen bolts from the pump head, and remove the pump head. Note there will (should) be a large rubber ring, behind the pump head. Have a suitable receptacle handy, as you will lose around 1/2 pint of fluid.
Strip down the new pump, as per the old pump. At this stage, it's easier to re-wire the new pump, as you can manipulate the new head to your advantage. Check the pump body internally. There should be a raised integral metal ring, that butts up against the pump impeller. Using the new rubber ring (about 3" diameter), replace the new pump head, onto the old valve body. Hand tighten the bolts only, and then tighten the bolts diagonally, using the Allen key.
Re-open the 2 large valves, that abut the pump body. The system can now be switched back on.
If in doubt, please contact a suitably qualified plumber/electrician.
Regards,
Ian