Hi Nicola
Are your walls single leaf or cavity?
Timber cills (and cills in other materials) generally have a groove machined into the underside to stop water that collects on the underside of the cill travelling back into the wall and causing damp problems. Other ways around this (eg in the case of brick or tile cills) might include laying the bricks at an angle so the water drips of the edge and can't run back (upwards). What evidence do you have that the cills may be causing a problem? Is there any obvious damp penetration of the wall externally? Or internally? Is mould / damp localised around the area beneath the window? (Now I need to go and re read your previous post which probably answere several of my questions...) julian

Thank you Julian.
to answer your questions:
I don't know if its single leaf or cavity, I guess its cavity (neighbours in similar houses, 1950s 3 bed semi, had it insulated I think).
 
I shall rule wood out then.
 
I don't know it the cills are the only cause, but I gather they are a contributing factor, as the jar-test (run water over cill and see if that hit the wall) proved it to be. (had originally overflowing soak away outside, via insurance they came 2x 2008 and 2009 to repair, seems fixed. Have since 2008 damage to ground-floor kitchen walls on the other side of the soak away; since 2009 the bed-room above mould and dampness are beginning to show.)
 
I want to see that I make sure I am not just fixing one thing (gutters) and have problems persisting or new problems coming - in other words: I think the house deserves a proper job (done so fare pretty well, with installing wooden floors, treated with Auro, and walls painted with Earthborne, both are great, and I can recommend them to everybody, overall bit-by-bit trying to get rid of voc-laden materials).
 
The rounded bricks used as window-cills, have now groves - and by the looks of it never had -
 
So my questions remain:
1. do I have to knock out the old window-sills (which would mean taking the windows out too I suppose?)
 
2. or could I put something (what?) on top of the old sills? - but this would then touch the aluminium frame (would that be detrimental to the misting of the windows? or anything else for that matter?)
 
3. or could I stick something underneath the old sills (what?), to do the trick? (since the sills are thicker than normal bricks, it would be unlikely to succeed with trying to drill grooves underneath them, also they only stick out from the wall about 1.5 to 2 cm.)
 
Any help on any of the above most appreciated.
Nicola

you could stick any sill to the bottom of the window with silicone and make it drip the water clear of the wall. A European sill or a flat piece of ali sheet laid to a fall or slate etc, even a piece of lead might do it.