Quick update on the wine making. It's been bubbling away quite happily sat in my airing cupboard where the boiler is. The temperature is usually a couple of degrees above or below 20 degrees Celcius which is at the lower end of what the instructions advise. Can't say it seems to be having an adverse affect though.
So yesterday being Day 3 was topping the demijohn up to 4.5 litres. There is one problem with this... Unless you know where 4.5 litres comes to on your demijohn you don't actually know how much water to add. You know you originally put in 1.8 litres of water, but you don't know what the contents of the can of grape gloop adds as it is measured be weight instead of volume. I've estimated it at around half a litre, so aimed to top up with just over 2 litres. The one thing to plan is that you have to boil the water but then let it cool. I assume this is so that you don't inadvertantly take the temperature of the wine too high and kill the yeast. I found it took around 4 hours to get to a temperature where I was happy to add it to the demijohn (the water was boiled in a stainless steel pan on a gas hob).
Checking again this morning and I still have a fairly vigorous fermentation going on. The wine now looks like a cloudy apple juice, or perhaps a real cider. I will keep monitoring over the next few days, but it will mostly be a waiting game now as the instructions say it would be around Day 10 (January 20th) before the fermentation is complete.