Scupper

Finally... I've done it!
 
Here's what you have to do, and remember this is nothing to do with any route or arrival procedure that you have programmed into the FMC: I think I was getting mixed up with that. In the end I just set up the nav properly in the Primary Flight Display (PFD) and hit the right buttons in the autopilot controls. Jim's advice in the post above this clarified for me what the Preset does (the clue is in the name).
 
First tune VOR1 in the TUN page of the CDU (flight computer) to whatever the ILS frequency is, then head towards a point on the extended runway centreline far enough away so that you can have a long final approach and intercept the glideslope (GS) from below at a higher height than normal (to give you lots of time to stabilise your speed during your approach). Use Heading Hold for this, (with the autopilot on). I tuned VOR2 to the same frequency but that step most probably isn't necessary. Make sure that VOR1 is selected in the PFD display below the attitude indicator/artificial horizon. Note, as Jim says, that while the "Preset" indication will say VOR2 at this point (or LOC2), the display shows VOR1 or LOC1, which is what you want. Adjust the course using the round knob on the grey vertical panel thing to the right of the PFD (the tooltip will confirm that you are selecting the course on NAV1) and aim to intercept at about a 30 degree angle on Heading Hold (e.g. if the runway is 27, intercept the localiser at about 240 or 300 degrees).
 
Soon the horizontal and vertical deflection bars will appear on the PFD, showing that the ILS signal is being received. When the ILS deflection needle on the HSI starts to move towards the centre, hit the NAV button on the autopilot panel. this changes the mode from HDG to LOC1, the localiser.  When the localiser is more or less centred, hit the VNAV button on the autopilot panel. At this point you can still be on ALTS (altitude hold) because the autopilot will override that when it intercepts the GS. Now hit APPR on the autopilot panel, which engages both flight computers. At this point, accross the top of the PFD you should see APPR LOC1 AP in green capital letters. As the glideslope bar starts to fall down the verical ILS deflection bar, you will also see GS above the PFD in green, - which means you've successfully captured the glideslope.
 
Now you just need to control the speed with the levers and put the flaps and gear down (assuming you're a lazy pilot like me who doesn't bother too much with pre-landing checklists).
 
Thanks to all who posted on this thread, and if you have any more advice, perhaps how to do it better, I'm sure it will be well received. 

Sorted. I opened Task Manager, selected the program, right-clicked and chose 'Maximise'. Will know if it happens again...