The missing number is achievable annual rent. Without that, nobody can tell whether your expense assumptions are harsh or the purchase price simply bears no relation to rental income.
One caution on treating the service charge as the main variable: the amount alone is incomplete. You need to know what it covers and whether the buildings are genuinely comparable. Otherwise you may be grouping unlike properties and calling the difference a market signal.
The asking price is roughly £426 per sq ft, but I would not apply a standard floor-area percentage from there. Start with the completed sale and establish whether it is genuinely comparable in building, street and layout. Asking prices are supporting evidence, not proof of value.
For condition...