First thing is to have a look at your lease – if its a 6th edn ADLS lease form then you should have a right (clause 27.5) to only pay a fair portion of your rental.
If you dont have a rent abatement clause then you will need to negotiate for a retn abatement.
I have negotiated 50% to 80% rent reductions for clients while agreeing to pay the rates and insurance opex.
Be open with your landlord about your cashflow. It is in your landlord’s interest to keep you as a tenant. You may need to work with both your landlord and the landlord’s bankers to negotiate both a rent rebate and a mortgage holiday so that both your and your landlord’s cashflows are aligned.
Also consider what you can offer the landlord in terms of extra tenure etc to strike a deal that meets both parties needs.