Thanks a lot for all the advice.
Based on the advice given my revised "instructions" are as follows:
NIGHT BEFORE
Get all equipment out and check it is all there and in working order.
MASH
1) Add a crushed camden tablet to 30 litres of water in a spare fermentation bucket and stir - this will be the source of all water for my brew. In future I might want to consider adding 8-10g gypsum, but I don't have any at the moment.
2) Preheat mash tun by adding a couple of litres of boiling water, leave for 10 minutes, give it a swirl, and empty it.
3) Add 10 litres of water to mash tun @72C
4) Add all the grains and "dough in."
5) Check mash temp is between 66-67C.
6) Fit lid.
7) Check temperature after 20 mins and add hot water as required. Refit lid and leave for a further 40 minutes
Stir mash and take temperature - hope it is OK.
SPARGE
1) Recirculate first runnings into mash tun.
2) Run the rest of the wort into the boiler but stop before grains in mash tun settle and compact...they should always remain floating, keep a few mm of water above the grains.
3) Cover grains with fine mesh.
4) Slowly pour water at 80-85C over grains and collect runnings in boiler. Stop when a sample of the runnings is 1006, see equipment section for temperature correction graph.
BOIL
1) Switch on boiler.
2) Once boil has started and the vigerous foam has subsided add bittering hops (50g Goldings)
3) 45 mins into the boil add flavour hops (20g Goldings)
4) 55 mins add whirlfloc tablet.
5) 1 hour add aroma hops (14g Goldings) and switch off boiler.
6) Cool wort - using immersion chiller.
7) Open boiler tap and let wort flow into fermentation bucket. Aerate and pitch yeast.
If I don't manage to get the immersion chiller working my next best option is to wait until the wort has cooled enough to safely run it int the fermenter, put a sterilised lid on the fermenter and leave it outside overnight before pitching the yeast.
I'm still not quite sure about how much water I use to sparge. Do I just sparge until the OG of the runnings falls to 1006 and let that determine the amount of wort that I collect. I'm aiming for a 25 litre batch, but I could end up with 20 or 30 litres depending on the efficiency of the mash?
and I wouldn't worry about the tap water, that little amount of water in that volume of wort shouldn't make much of a difference, you may have lost a little gravity, but i wouldn't worry about the chlorine/chloramines.
The beer was named Bathtub Bitter, of course.