The Bolam test is increasingly applied to less and less intricate skills though. Whilst the Bolam test started out as medical only it's now used in pretty much any tortious situation where a specialist skill is held. A quick search shows up it being used in:
A v Hoare; H v Suffolk County Council; X and Y v Wandsworth London Borough - a teacher
R (on the application of Bramall) v The Law Society - a solicitor
Carty v London Borough of Croydon - local education authority
Great North Eastern Railway Limited v Hart and others - proffessional highway and bridge designers
etc.
It's also worth mentioning that in this scenario it wouldn't be used against the council workers who installed the manhole, but the person that decided that type of manhole cover should be used - which is probably some civil servant as it'll probably have been decided centrally for all council civil engineering jobs. As such, it is to say that the procurement of materials for council civil engineering jobs is a pretty specific skill set.