dartgod - many thanks, both posts helped. I did just pour in the topping-up water then took an immediate reading (without stirring). I reckon I added about 4 litres of top-up but the 19 litres of wort, before top up, was just about 'bob-on' at 1040 so there's no doubt it is weaker than it should be. But perhaps not as bad as I first thought.
I like the idea of checking pre-boil OG - you use beersmith but is there a simple way of calculating this, do you know?
In the end, though, I just don't think I built into my calculations enough allowance for losses in the process - this was the first time I hadn't meticulously followed a recipe
