Non fault call outs are now only twenty five quid; Sadly you still need to book it by dealing with VM's awful customer services (unless the forum staff can book these, I'll flag the post for their attention). Ideally the faulty V6 needs a visit to fix, and doing the two together would make sense, not sure that VM will make it easy to book joint fault + non-fault call outs at the same time. If you're after a Gig 1 connection, you might want to see if that can be done at the same time?
Obviously the "in contract" upgrade price won't be as good as a well fought retentions offer, on the other hand if the retentions agent can see you're angling for 1 Gbps and you've paid for the property cabling to be changed they really ought to be offering a minimal discount because the facts suggest you are unlikely to up sticks and go back to Openreach.