Many VM installations are like this (some even surface laid), but when they get builders to do it on a new estate VM require at least 250mm of cover and installation in a rigid wall 54mm duct. If VM's cable is damaged because it wasn't properly buried (in ducting and at a sensible depth) then they're at fault. Let them replace it, if they do impose a £25 charge that'll go through on direct debit I assume, you then raise a formal complaint asking for a refund because the principle cause of this incident was shoddy installation.
The complaint will very likely be fobbed off, but don't worry about that....you then let things ride for eight weeks from the date of your complaint to VM, and ask the fine fellows at CISAS for a resolution that includes (a) refund of the £25, (b) compensation for wrongfully charging £25 for fixing a situation of their own making, and (c) poor complaint handling. Should get around £40 for each of (b) and (c) as well as the 25 quid back.
If you were feeling stroppy you could ask CISAS to make VM come back and relay the cable properly for its full length, but that might be a very high risk strategy given VM's track record on cable laying and replacement.