So you need that actual valve block modulator part....not the ECU?
I know the ECUs can go bandy (mine did) not heard much about the modulator going south before (not to say it can't).
What makes them sure it is the modulator needed to be changed......ABS ECU going nuts will throw allsorts of spurious faults.
The main issue with the ABS ECU is two things: a) the Harness suffers from water ingress and can corrode the pins in the connection (LR issued a Tech Bulletin about it and the solution is to fit a new 'Short' harness - I did this to mine, still had issues....and b) where it is located above the exhaust manifold it gets hot and suffers heat damage internally...this can lead to the tiny little wires inside working free from the circuit board giving odd errors and faults.....search my threads for mine as I opened my old ECU up to take a look! it is well reported in the BMW models too.....
I'd be tempted to ask them if they have a spare ECU they could try and if that resolves the issue, replace the ECU at a fraction of the £2k cost....I paid £260 for mine from Ebay.....if it does in fact turn out to be the modulator...then you have two options
a) second hand - Ebay most likely or a decent breakers like Furness or Emmots of Colne.....but as it is a major component of the braking system, I'd think carefully about an unknown second hand unit from anyone who is not a reputable company selling parts.....try stopping 2.5 tonnes of Solihuls finest on a wet raod and you'll see what I mean!!
or
b) a new part and get financially ruined for the price!