ABS system failure

Hi All, I have recently had an issue with my 63 plate A3 where the brakes have been putting themselves on (suddenly and harshly!) above 60mph. This has been intermittent but more regular at higher speeds, eg 70mph. It’s very dangerous when it happens and there is no warning. No dashboard lights, no warning in how it’s driving etc etc. The car slams its breaks on and then all steering goes, which is very dangerous on dual carriageways / motorways where it happens. I took the car to my local garage and they said it’s likely to be ABS and to send it to Audi. Sent it to Audi and they did 1st stage testing, thought it might be a master cylinder so replaced at a cost of £500 or so to then test again and that not be the problem. They are now telling me that stage 2 testing shows that the whole ABS system will need replacing at a cost of over £2000!! I’m finding it hard to believe my whole ABs system needs replacing after 5 years! Anyone else experienced similar? Is this right? Thanks in advance.