A bit of confusion here the £1735 inc vat for is for a secondhand reconditioned box, (uncertain what reconditioning is done).
Good you got a quote from Dartford for a recon unit, although I think X5sport was suggesting you got a quote for yours to be fixed.
Richard at Barkers quoted £1800 plus vat for brand new ZF supplied box equating to £2160 inc vat......For me, (if looking at replacements only), then it's a no brainer & I would go for the new box - even if a little more expensive. I would only consider a repair if able to achieve same for a significant saving over a replacment box.
I thought Richard would be competitive as I prior discussed box costs with him when we spoke re expenses involved to deal with such failures on our cars.
If you check with main dealer you will see what a good price he has quoted..........
Richard used to be a head or lead technician at BMW so knows his stuff - he also actually owns an E53 X5 & comes across as both decent & genuine.
Just to add that two indies I know locally to you have prior said they use a couple of specialists
link1 and
link 2 .........Maybe worth a couple of calls albeit I doubt you will get better new price than from Richard.
Good Luck whatever you decide...............Ideally don't leave it too long to decide or you might end up with extra costs to get car to chosen workshop on a trailer.