He said that the latest regulations say it's got to be 2 metres from
a boundary (boiler would be on party wall so no chance with Worcester 35
Cdi as planned)

Get another plumber. He would never get the job in my house. It is 4m wide,
so presumably he thinks that all gas boilers appliances are banned in
Victorian terraces, as there is not a single point anywhere on the property
more than 2m from a boundary.

Christian.