I'd recommend NFU's service unreservedly - been with them for motor insurance for 13 years.
I have no experience with their business cover, but they do have local offices though (for claims, etc.), so worst case you can probably talk to someone in person.
Whoever you insure with, if you want more than public liability, expect to have to meet a lot of safety standards, for electrics, dust/fire, chemicals storage etc. and they might want to know about the legality of working in a residential area (if you are). All the safety stuff is important anyway, and there's no point in paying the premium only to have a claim refused when you really need the cover.
You're probably on top of all this - just sayin' in case.
E.