In this months (June'20) Waterways World a reasonably priced Remote Battery Monitor was advertised. It also has the advantage that it sends location information along with the battery charge.
I've ordered one from the manufacturer ChargeMetrix (£42.50 + a monthly connection fee of £3.99) and will install it once the lockdown is ended and I am able to visit the boat again.
Although it's too late for the Coronavirus lockdown it will be convenient to keep an eye on the battery charge in the future and if ever the boat is stolen be able to track it.