I've been thinking about options to cover all basis and came up with this. To allow guests to use a bath/shower the ensuite would be shared. I added a wall in the master bedroom to divide it and add privacy so guests don't have to walk into the bedroom to get to the bath/shower. The upsides are that the ensuite is larger, includes a bath and shower and has more vanity room. Also, by diving the bedroom I can add sort of a walk-in wardrobe. The downsides are that there's only one bath/shower that has to be shared and guests have to walk through the walk-in wardrobe to get to it. Also, it reduces the size of the master bedroom but it's quite a long room anyway and it would cut out the light from the windows in the bedroom. Personally I don't see these as big problems but I guess some people won't like it. Do you think this is a good idea?

Thanks for all the great ideas guys! Just had the builder to view the flat and here are some updates: builder said the soil pipe seems to be straight down, but could install a pump to flush it; although the estate agent said the pipe goes from external wall at the back of the flat. Not exactly sure who is correct. the budget is likely to go between 30k-35k. I also had a measure of the current bathroom dimension which is 1.67*2.13m. and the ceiling height is 2.56m. we'll probably change the bedroom wardrobe and free up some space as well...

If you shop for a space saving toilet there should be plenty of room for a full sized basin on the same wall. A p shaped bath would also give you a little more room at the shower end.

Thanks pjbell! My first thought was 'no way, we use bedroom 2 and can't lose it - far too radical'. But then I stopped and looked and actually, that may well be worth considering! All bedrooms are in use - bedroom 2 is our studio/craft room/office. Kids are in the back bedroom and top floor. I've got my heart set on an en-suite and keeping our large bedroom but have been mindful of making the studio too small so was opting for a small shower room. I've been pondering if there is anything we could do to have a larger family bathroom but couldn't see how. This could give a large family bathroom and more luxurious en-suite. The back room is south facing so would be much better for a studio - its worth seeing the size if we split into two bedrooms. I do like the fact that all bedrooms are decent doubles and wouldn't want them much smaller. We are going to be here for the next 10-15 years and will then look to downsize when the kids have flown the nest. This will be the only major work we'll do so although it needs to be right for us, I also want it to be as appealing as possible to the next family. Definitely food for thought - thanks!