Thanks for all the information mate - will probably go with replacement windows as we are working to a tight budget and already have a back door which gives good access out the back.

Just one further clarification :

'For replacement windows, you either have to have the work done by a FENSA
certified contractor, or DIY or use an unregistered contractor, and get a
building warrant'

Does this mean all window replacements need a building warrant or only those done DIY or by an unregistered contractor ie if we seek out and use a FENSA certified contractor does that do away with the requirement of a building warrant?

As we already have a window of the same size / in same position I can't see why planning permission would be an issue at all but will check for my own peace of mind.