Is the cost of a CRB check for a "self-employed" teacher tax deductible

I recently did some teaching work in an FE college. Belatedly, the agency which solicited my services issued me with a contract that says I am self-employed. I disagree that I am self - employed as the work I did on this temporary contract is almost identical to the work I did when I was employed full time to teach in FE colleges. I have asked HMRC for a ruling on my status but their reply does not provide an answer despite me setting out the circumstances and a copy of the terms and agreement upon which the agency engaged me.

To undertake the college work I had to obtain an enhanced CRB check. The legislation demands this. Also, the CRB check is not portable - it applies soley for the college and role I occupied. Hence, it seems to me, the CRB check is necessarily, wholly and exclusively incurred in the course of my business. HMRC says it will not allow me the cost of the CRB check because it was not incurred in the performancy of (my) duties. Is this a correct reading of the legislation?

The agency also deducted so-called indemnity insurance from my gross pay. The deduction is a percentage of my gross pay so that I only pay the indemnity insurance when I am earning. HMRC says this cost may be allowable and that I should seek advice. Again, I had no choice about the matter - I had to pay it. It is a wholly, exclusively and necessarily incurred cost. I would not be permitted to undertake the work unless I had cover. Surely, because it is a variable cost, ie it increases in sympathy with my earnings (or with time spent teaching), the cost of this so-called indemnity insurance must unambiguously be incurred in the performance of my duties?. Could someone confirm this is tax deductible please?

One final point, both of these costs were incurred wholly, exclusively and necessarily in relation to the work. My understanding is, therefore, that it does not matter whether I am an employee or self-employed for these costs to be tax deductible. However, there is the possibility that I may be a "worker" which may mean I am neither employee or self-employed. If it is possible for individuals to be neither then there is a chance that the legislation applying to employees and self employed does not apply in this case. Can anyone advise please?