Uk water shouldn't have chloramide in it, only chlorine, so water changes are pretty simple (especially with no fish to worry about). Its easier with a siphon - often called 'gravel cleaner' in the shops - but can be done with a jug if you don't have one. You will also need a 'fish use only' bucket (or other large container) - to make sure you don't get bleach etc in your tank by mistake...
1) Siphon out tank water into the bucket. For a 40 gal tank you probably want to take out 1 or 2 normal sized household buckets (15-30 litres) depending on how often you plan to do this. If needed wash your filter pads in this water (once a month?), then throw away the water. I have the same size tank - I do ~25 litres once a week.
2) Fill bucket with tap water (cold) - add a little hot from the kettle (hot tap water often has bad things in it from standing warm - although many people seem to use it no problem or indeed add no hot at all and let the heater deal with it) to bring temp up to 'near tank'
3) Add appropriate amount of de-chlorinator - swirl it about a bit. There is no need to 'stand' if you use dechlorinator.
4) Pour into tank (siphon is better if you have fish in the tank, but you'll need something high to stand the bucket on - which I don't so my fish have just learnt to like the bubbles)
5) repeat steps 2-4 as needed till tank is full again.
So to answer your question you can add that water from the spare tank anytime. The only reason I can think of you might 'stand' water after adding dechlorinator is to let the temp raise naturally, rather than adding hot.
Hope thats everything...
aj xx
