By main blue fuse do you mean the RCD? The isolator with a fan on it, is for isolating extractor fans. It seems someone has installed to isolate the loft and is not rated to be used on socket circuits, without being downfused first. As it seems to happen when the shower is in use, I would conclude, that may be the pump is packing in.
Can't find your shower listed but replacement says Power ratings: 230v AC@50Hz/3Amp/150W I would assume 3A inrush as 150W more like 1.2 amp.
I would assume blue = 16A as Wylex used blue to show that rating.
This does raise some questions as with standard ceiling roses the maximum size of supply fuse/MCB would be 5/6 amp not 15/16 amp as the ceiling rose is rated either 5 or 6 amp only.
Fan isolators are normally 6 or 10 amp rating which again raises overload questions.
The fitting of sockets on lighting circuits is not really compliant with "314 DIVISION OF INSTALLATION" in that "(iii) take account of danger that may arise from the failure of a single circuit such as a lighting circuit" is not really complied with.
In real terms there should not be a problem but electricians now have to submit paperwork for what they do so breaking regulations could come back and bit them.
If it were my house I would install an array of fuses or switches so I could with ease isolate areas to find what was causing the problem. But for you much depends on what you are happy DIYing?
We clearly should tell you to get an electrician. But I fear the sharp intake of breath and jobs worth. And it really is jobs worth if an electrician loses his self cert status he may as well give up.
As already said most likely is the shower pump but question has to be how much are you willing to do as DIY?
The isolator switch now trips the blue mains RCD on MCB (if that is the correct terminology) every time I turn it on, so can't have attic/shower running at all.
Not sure why the previous installation wired that isolation switch to the whole attic - most annoying as the TV aerial amp is wired in up there, so no TV reception until this is fixed !
I will get the electrician to isolate the shower cubicle properly, and also look at wiring of the house to check for other anomalies. I suspect Billy Bodge has been at work in this house sometime previously ...