Paul,
They are extreamly easy to change, just be carefull not to overtighten the hose-tails - they are a tapered thread.
If it is a demand pump (identifiable by the pressure switch at the top), one of the main causes of failure is the switch itself. This is available as a separate part, and easy to change. To test, disconnect from mains, disconnect the two black wires from the pressure switch and connect them together. reconnect to mains, switch on pump and if it runs, renew switch. if stil not running the motor is faulty - renew pump.
If a bypass pump (no pressure switch), and the pump runs without pumping, the 'poppel' valves are a common cause of failure. These are little plastic 'mushrooms' located within the pump-head.
Poppel valves, diaphragm and a couple of other parts are available as an overhaul kit. Kits are available for both types of pump. All the above parts are available from most pump suppliers.
If you need to get the machine up and running quickly and decide to buy a new pump, it is still worth repairing the old one so that you have a standby.
John.