Seized injector pumps

Probably best to leave them to soak until they free off, but a bit of gentle heat applied directly to only to the top of the cast-iron pump bodies (around the delivery valve holder) with either a big electric soldering iron or a heat gun wouldn't do any harm, and might just be enough to do the trick after soaking in diesel for a bit longer. All the numerous internal parts of each pump are heat treated/hardened, including the parts which won't have been affected by corrosion and are re-useable, so applying any serious heat to them is to risk drawing the temper or softening various small internal parts of the pumps to the point where they're only fit for scrap.

Diesel fuel is hygroscopic - it absorbs water vapour from the atmosphere - and the single element jerk pumps used on the Lister 'S' range engines have a small recess in the pump body extending to just below the inlet and spill ports of the element barrel where the absorbed water can settle out whilst an engine stands idle and then cause corrosion seizure in the pump barrels and plungers. I suspect this is what's happened with the pumps on your SL.  

Once freed off the pumps will probably work reasonably well if put back on the engine, re-timed and re-balanced by way of exhaust port temperatures instead of the calibration marks on the racks, but the corrosion damage to the pumping elements, and probably the delivery valves too, will almost certainly leave them performing below par, and engine performance - starting, exhaust smoke etc. - will be affected to some extent.