Gaining access

Access

I am needing to sell my 3 bed HMO and I have access to all areas apart from 1 of the bedrooms where the lady is not being responsive to my access needs. Instead of serving her notice I’d prefer to look at other legal routes. I spoke to the estate agent and they have it written in their agreements that ‘they use’ that grants them access if you have given the tenant 24 hours notice. I can see that within the NLA tenancy it says something very similar.
Would I be breaking the law if I did this? According to the AST I’m within my rights for valuation and viewings with 24 hours notice given. Secondly how is it best to serve this notice. Email and text together with attempted call?
I’d prefer to do all this within the access terms. Thanks in advance