Dear @judibutler I assume you are referring to a wall which is wholly above ground and that you have completely ruled out damp tracking into the wall from above. A lot depends on the size and thickness of the wall and other factors, such as if the building is listed. Damp penetration (driving rain) through granite walls has been an ongoing problem in church towers in the southwest of England for some time. Ironically the problem often seems to get worse after re-pointing in lime. Granite is fairly impervious and was often in laid in large blocks meaning trapped water has little chance of getting out through thin mortar joints. Historic England held an excellent conference on the subject in 2013; the transcript of proceedings can be found at: http://content.historicengland.org.uk/content/docs/research/damp-towers-conf2013-programme-transcripts.pdf A roughcast lime render coat may help, but there could equally be voids in the wall which are holding damp and may need grouting. However, grouting is a very tricky and expensive technique which should not be undertaken lightly, and only works if one knows exactly where the voids are. Over-cladding the whole wall with semi-sacrificial weather boarding may be a last resort. This and rendering are likely to require planning consent. Depending on where you are located, a company like ArchiMetric (www.archimetrics.co.uk) could insert some interstitial moisture monitors into the wall to try and find out exactly where and when the damp is getting in and how it is moving through the wall, but this may be expensive. Some localised investigation by dismantling pockets of stone may be useful. The Society of Ancient Buildings helpline (mornings only on 020 7456 0916) may be able to help with specialists in your area. Most importantly try to get an understanding of how water is getting into and moving through your wall before you embark on anything. If your problems relate to below-ground damp then a different response will be needed.

Where does the boxing on the floor (left) go to? The basin? Water and drain? What's behind the left wall? For example: our basin drain goes through the wall and is hidden behind the kitchen filler panel, then goes back into the toilet space behind the loo, where the soil pipe is. We have a vertical box too (upstairs bathroom), but I don't mind (it's nicely squared off though). If space allows, pull the loo forward and opt for the deeper sill, or somewhere below it.

Thanks for all your input and the wonderful diagram! I agree that ideally the bath and loo would be swapped. I need to talk to the plumber about how feasible / expensive that would be if it is prohibitively expensive / difficult I think I'll stick with my idea. I've not come up with any good reason not to have the shower in the centre of the bath rather than the end and I hope this might actually give a bit more room when in the shower rather than being squashed up at one end

You made me laugh out loud there rinq! Yeah...that one's been ruled out already :-) Here's a pic as it is at the mo: We did half an update around 10 years ago - new bath/screen/sink/taps/cupboard doors/floor - but our budget didn't stretch to retiling. The main drivers today are: to get a toilet in here new tiles (wall & floor and U/F heating) TBH, I want it to look more modern and I want a toilet. If we have to do without the separate shower, so be it.