Many thanks for your helpful reply. I can't do much until Saturday but I am going to give it a go then. I'll report back what I find.
Cheers
Progress report and a possibly significant finding
Since this is an intermittent fault, and as the light is currently not coming on (typical!), I decided to follow up troubleshooting leads from many posts on the subject. Hopefully I would find something significant. Here is what I have done so far:
Brake Fluid
1) Checked level and it is exactly at MAX with engine running and a few pumps of pedal.
2) Checked and cleaned float under filler cap - moves and clicks smoothly.
3) Removed, cleaned and reseated connectors on top of filler cap.
Battery
Checked voltage of battery. Ignition off - 12.34V. Engine idling - 13.96V. Battery warning light is on when key is turned but goes out immediately engine starts. Does not come on when idling. Figure that's OK.
ABS Power Supply
The ABS system gets its supply from a second lead attached to the +ve battery terminal. Some say this can come loose, fray and/or corrode. I removed the terminal connector, cleaned the connection, checked lead was in good condition and replaced - tightening firmly.
R34 Anti-lock Relay
Started car up. ABS light comes on when ignition is turned but goes out a few seconds after engine starts. I removed the relay and re-started engine. ABS light stayed on and alarm sounded. Put the relay back in. Again light stayed off/no alarm when engine started.
To duplicate hitting a bump: with the engine running I wiggled, lifted, pressed down on, tapped sharply, and pushed relay from side to side. It rocked quite easily but I was unable to make the light come on - i.e. connectors seem well seated. To be on the safe side I removed relay, cleaned connectors and reseated it firmly.
Front wheel ABS sensors
1) Uncoupled the white front sensor connectors from the black cylinder. Tested the resistance across the terminals in each front sensor. Results: 1.044 KOhms on right front and 1.041 KOhms on left front. Unless anyone says different - I take it that the two sensors are working correctly as these figures are all but identical to each other and in the range reported elsewhere. Reports of faulty sensors state that resistance shoots up to figures like 27 KOhms or 10-30 times normal.
2) While here, I removed, cleaned and reseated the connectors for the brake wear sensors on each side.
3) Unable to remove and clean the sensor grounds on either hub. The hex nut, which holds the sensor on, and which also bolts the ground to the hub (brown lead), was well rusted and could not be moved. (same on both wheels). I did not want to end up breaking it off as has happened to OPs with same problem. If I have to change the sensors then I'll have to break them.
4) When I came to reconnect the right sensor plug I found what maybe a significant development. One of the metal tubes which forms the female side of one of the connectors (where the male plugs in) had sheared off and was lying loose in the plastic plug. It fell out when I was measuring the resistance. This "tube" should connect to a piece of metal at the back of the plastic plug but it is possible that the connection was not being made reliably - hence an intermittent fault.
The first photo shows the two plugs (wear-sensor and ABS sensor). The female parts of the connectors can just be seen in the left-hand plug. The second photo shows the broken piece of the metal connector.
This plug is welded to the short piece of cable and presumably is part of the ABS sensor assembly and could not be bought separately. I thought of trying to get one second-hand or otherwise repair this one but have not got much further than that yet.
I jammed a bit of silver paper down the hole and forced the broken piece of tube back in - then pushed the white plug back into the male as firmly as it would go.
I just can't get the abs light/alarm to come on today on a short drive. The task now I suppose is to drive the car until either the light comes back on or I presume it is temporarily fixed. (And in the meantime investigate a more permanent repair for the broken connector).
Other things not yet tried:
ABS Valve Unit
The whole of the boot (trunk) area is immaculate on my car - not even a scrap of dust. The valve unit looks brand new and I can't spot loose or dirty connectors inside it. OPs have said that this unit does not normally cause any problems so I have left it alone.
ABS Central Unit.
I have yet to identify this but am going to look again and check for dirty/corroded connectors here..
Thanks for all the help people have offered.