Freeholder difficult to communicate with and looking to sell?

I own a leasehold flat and which I presently Let. The property is a conversion 1930’s standard build split into two flats with own entrance.

As the lessee I have an obligation to pay the ground rent due. Over the past 5 years I have submitted my payment by cheque posting it through the freeholders letter box and requesting a receipt in return.

I do not knock any more as she simply does not open the door and when she does respond she opens her upper window and then simply asks what I want and then closes it again. I do not have her phone number, bank details to affect a transfer or otherwise.

I owe the Freeholder a total of £350 to date. I would like to sell the flat this coming summer and I am increasingly conscious of the difficulties that this might create. I am thinking of going to a solicitor and ask them to send the payment with covering letter on my behalf as proof, but even if I do this and she decides not to cash it then it still leaves me with non payment of ground rent.

Ideas and suggestions please!

Saul