Just to put my two penny worth in on this thread, and having extensive experience in obtaining estimates from builders on a commercial basis, I hope Sir Tigger you are following the basic rules of finding the best contractors, who are often not the cheapest.
I could type away going through the "check list" of things to do and not do, but instead being lazy just click on this from the National Federation of Builders which gives you an excellent guide:
https://www.builders.org.uk/find-a-builder/tips-for-choosing-a-builder/

Was that before or after your vast accumulated expertise in retail Lizzie? 
No, actually at the same time. Large retail premises, or even smaller ones, along with many associated properties, require continual investment in their structure, alongside regular repair, and every 5 years refits. Knowing your contractors for jobs sometimes going into the tens of thousands of pounds was vital, as was knowing what had to be done within a very strict time scale. Meeting local authority requirements, and Health and Safety objectives was all part of that, and only the best builders were usually employed, with full references and Tax Deduction Certificates that we were required to check.
As I made clear in the tread you are referring to, but obviously did not read fully, I was responsible for all aspects of a multi-million pound division and it's P&L. I wore many professional hats to achieve that, but thank goodness I had a property manager to call on who was fully qualified in that specialist field!