Paul,
To answer your question; I believe the answer to your OP question is yes (if you are self employed). If you are an employee then you should receive an allowance for personal expenditure in pursuit of your work (equivalent to getting private mileage).
You really need to speak to an accountant who should, initially give you the general answers you need as part of the opening "can we do business" discussion.
Alternatively there are pages of information of the web and a good place to start is:-
HM Revenue & Customs: Home Page
Also, I think you should phone up the HMR&C employees helpline again and explain the 3 answers you have received and ask for verification. Should you then receive an affirmative answer to your OP question then take down the call reference number and quote it in future should there be any dispute.
As you are aware I am a temporarily retired non DIR diver so you really can't trust anything I say.
Still hoping to become Comfortably Numb but feel more Dazed & Confused.