Whether or not the cause was outside VM's control, that's something that doesn't stop them being liable for automatic compensation. These are the Ofcom rules regarding automatic compensation. Read section 40 and 41, or which the relevant part for you is "the Communications Provider does not avoid payment of automatic compensation if the issue was caused by an event outside of the customer’s or its own control". Sounds harsh, but that's because it is up to the ISP to do proper surveys and commit to realistic dates.
If VM still won't accept that they're liable (they have form for attempting to evade their obligations to pay compensation on delayed installations) then wait for your complaint to be fobbed off. It will be fobbed off, partly because complaint handling by VM is very poor, and partly because the company clearly don't understand the regulations that apply as per the link above. When you get the fob-off letter or email, you reject that resolution and ask for a deadlock letter. With the deadlock letter you ask CISAS to review VM's failure to abide by the scheme rules and to pay the relevant £440 odd quid, plus an additional £50 for the poor complaint handling.