Morning Adrian
Your thread has been read 80 times without reply which might indicate that your question is very difficult to answer.
I see all sorts of daily rates quoted. Some tiny, some huge, many in between.
You say that you have 25 years experience in construction and FM, but with no indication of what that experience has meant.
For all I know you could have spent 25 years installing fire alarms and may be an expert on how many fire alarms should be in a building, to what specification and in what locations - but you could offer very little value in terms of providing a broader range of H&S advisory services in construction, FM or beyond.
So, at the very least, you need to give the readers a better picture of your skills, knowledge and experience.
Ultimately, you need to consider how many hours a week (INCLUDING travelling and work at home, in hotels etc) you want to do and how much you want to earn per week, and assess how you sell yourself to the market for that amount or more, making sure that you include for expenses OR keep them separate from the daily rate.
AND you need to include for all the other costs that becoming a consultant may involve - so you might be spending more time at home and running up higher energy costs, you might want to pay an accountant, get in somebody to market you etc etc.
AND it may be entirely appropriate to set different rates according to the value of what you offering.
As example, I used to work for a major engineering consultancy - in general hourly rates were set based on the individual's annual salary (divided by circa 2000 hours per year) and then multiplied assuming a ball park of a multiplier of 2ish simply to break even (LOTS of backroom costs in a large organisation, you should have lesser such costs).
.....but then the bid team would consider how much ABOVE break even to be bidding at and the number would vary enormously AND of course the price included for at least part of the cost of actually putting together the bid (Win some, lose some, but the bid team have to be paid)
So, on a huge contract, the multiplier would be set at fairly close to break even - high volume, low margin.
For niche work but a small contract, a MUCH high multiplier. For expert witness work or anything similar such that there is very little competition (so perhaps someone on the "ICE Panel" of reservoir engineers - no, I am not on that Panel!!), almost whatever number you choose..