I've got a non O/D SIV. Will a Humber scepter O/D gearbox be a sensible replacement re ratio's, mechanical fit etc? will I need to use a different prop shaft and back axle ratio? Any tips will be much appreciated

If it's an Arrow Sceptre - the Hunter shape, it won't fit (though you can use its parts to build up one which will).

If it's the 62-67 Super Minx type, it will fit. The ratios will probably be wider than Alpine, but OK.
You will need the shorter propshaft.
You will need the bulkhead relay and the column switch
You will have to sort the speedo. If yours is about 1020 turns/mile, it will probably read about 50% fast! A SIII Alpine speedo will be close if the Sceptre speedo gears are 5:12. The Sceptre speedo may be OK.
Most Alpines (except SIII) had a lower (4.22) axle ratio when OD was fitted, but I am pleased with my change to the non-OD 3.89 which is what you have. So leave it.

You don't say whether you have the later all-synchro box now. All synchro boxes have reverse to the right of 4th. Earlier boxes without synchro on 1st have reverse left of 1st. If you have the later box now, I would hesitate before fitting an earlier box, even with OD. Better to find a later OD box.

Thanks for all the info Chris, much appreciated.