As a result of a link on the 'Alternative unvented cylinder brands' thread to https://energy-surprises.blogspot.co.uk/2012/10/combi-boiler-versus-hot-water-tank.html - I went to the author's own website and came across her Simple Home Heating Model at https://nicola.qeng-ho.org/housemodel/interactive.php

It's a very simple model but it endeared itself to me because it let me select 'passivhaus' for the insulation levels so I thought it was worth posting a link to it here. Heat loss models are oft requested.

Nicola is interested to improve it, if we can think of anything and hopefully she will join in here.

The main factor that makes it inaccurate in my case is that it currently doesn't take account of solar gains. so that's one area where figuring out what might be an appropriately simple level of adjustment could be useful.