Step 1: Go to Halfords - Get a free battery check. If battery is faulty go to step 3.
Step 2: Observe how long the glowplug light stays lit when starting the vehicle, it should go out almost immediately after starting, if not then change them. It is wise to change them anyway if starting issues occur.
Step 3: Go to ECP website, look for batteries / glowplugs, insert forum discount (or offer discount if it is more)
Step 4: Buy parts and fit. (If fitting battery do not forget to buy/borrow a battery memory saver - stops the ECU resetting / Immobiliser kicking in)
Diesels are hardy little engines, and not a lot will cause them to not fire, short of not enough power on initial start up or the combustion charge not getting enough heat from a tired glow plug.