Hi Ian,

I'd start with seeing if "Fuel, electrical" is behaving.
(Dunno if there can be a faulty fuel line or fuel filter do this?)
You can use your DC voltmeter on Fuse 1 and Fuse 11 to see if the DC supply is
dropping to the fuel pumps when the engine quits.
Use the cigar lighter ground and probe either ATC fuse leg up top.

If the DC supply drops out then the fuel relay has a strong chance of being the culprit. It's easier to swap than to measure the relay in place. If your fuel relay is original I'd replace it with new.

Be aware that swapping relays can damage the socket pins. When rocking and tugging relays out of the socket you can spread open the pins and loosen the
contacts. You can make a gauge with the same thickness male spade lug
crimped and soldered onto a stiff 12 or 14 gauge wire to check the socket pins
before re-inserting the new relay.
Loose socket pins may be the cause of toasted relay sockets.
They can be removed and be tightened or replaced.

If DC voltage doesn't drop out, you can check each fuel pump by measuring
current flow at Fuse 1 (Both pumps) and Fuse 11 (in-tank primer pump)


Here's one way to measure fuel pump current flow with your VOM:
Compute current flow in ATC fused circuit by voltage drop.

Use this chart to calculate the current from the millivolts readings probed at the top legs of the ATC fuses in place.

https://m.roadkillcustoms.com/wp-content/technical-pdf/Standard%20ATC%20Fuse%20Voltage%20Drop%20Chart.pdf

Most VOMs measure up to 10 DC amps tops. To measure higher current
You can get an analog 1-50 DC ammeter with shunt at ebay and make a nice probe
out of a dead ATC fuse. see here:
https://www.brickboard.com/RWD/volvo/1687323/make_atc_fuse_socket_plug_dead.html

OR (This handy option has time limitations)
https://www.harborfreight.com/30-amp-automotive-fuse-circuit-tester-67724.html
"30 Amp Automotive Fuse Circuit Tester - Harbor Freight Tools
Product Overview This handy automotive fuse circuit tester has specially designed test leads that plug directly into your fuse box to quickly diagnose electrical problems. The tester can be used on all circuits up to 30 amps. Test automotive circuits at the fuse box, plugging into ATC blade-style fuse slots Simplify electrical diagnosis." Can you find HF in Scotland?

The ECU controls the fuel relay: It needs good inputs to work right.
Quality connections for signals and grounds are crucial.
You can look here for a lengthy look at troubleshooting LH 2.4 engine management
with many interesting items, including a specially modified fuel relay.

https://www.volvoclub.org.uk/faq/EngineFIComputer.html

Good luck, Bill