What about something along theses lines ..... [ marketing note - this is an example of the sketch consult service OnePlan provide... We happily provided this for free to this Houzzer, on this occasion . Normal rates would be £35 p/h. Full CAD images available too, at either an hourly rate or fixed room rate. We are freelance concept planners, selling only design !]

With the space you have available the easiest thing to do would be to take some of the landing space into the bathroom, split that into an ensuite for the larger rear bedroom and a main bathroom accessed from the landing. Then move the partition wall between the two front bedrooms and make them more similar in size. As long as this doesn't involved structural work this should be relatively easy. You won't get an extra bedroom but you'll get a better range of bedrooms and an ensuite. You could possible add a little more space into the rear bedroom when you reconfigure the bathroom as well to make this a better sized master bedroom, or add in a wardrobe space.

Could you post a photo of the back of the house please?

Thanks Jonathan for your plan, that is a plan we have been debating but think we are struggling to keep the bedrooms good sizes but at the same time have a good sized bathroom!