the relay is £15, the pump £250, so make sure you replace the right bit.
The relay is in the box of relays on front of the battery, easy enough to clean the contacts. The pump is DIY able, there main reasons it will not run is either the relay whic supplies the power, low on sele oil and the pump itself has gone.
The relay you are going to check out, if working you should be able to trace the wire to the pump, check it is live, a connector about 6 inches away from the pump is a good place to break open and get a 12 tester on the end.
Oil is easy enough, make sure it has some in and it's the right stuff.
Finally if there is power to the pump connector and no sign of life when opening the door, its the pump.
I'm not sure on a 156, but a 147 its just a matter or removing the front bumper to gain access to the pump, undo a couple of hydraulic lines and a few boltd holding it on and it's off. A 156 will not be too different.
It will not start in gear as a safety measure.