Hi Dave,
From having over 30 years experience in deep cleaning bathroom surfaces, i would recommend using a bathroom cleaner based on micelles technology (non-caustic,non-corrosive,non-toxic) in combination with microcleaning hand pads (magic eraser pads) this i have found to be the safest and most effective way to deep clean
tiles,grouting,sinks,baths,showers taps,etc.
Once the deep cleaning process has been carried out its just a matter of maintaining the surfaces with the micelles bathroom cleaner and bathroom microfiber cloths. All these technologies are available from janitorial suppliers in the UK.
As to the yellow staining on the grout lines; this more than likely has been caused by the continuous use of high alkaline bathroom cleaners which are caustic and corrosive based and probably have chemically burned the grouting
A good way to test this is to prespray the grouting with the micelles bathroom cleaner, allow it to dwell for 2 minutes, then tear a strip from a microcleaning pad and scrub vigorously in between the grout lines. If you cannot remove the yellow staining like this, then you know it has been chemically burned.
I must put my hand up here, for many years i used high alkaline based bathroom cleaners, i know from experience the damage they cause to bathroom surfaces from continuous use
To give you an idea of cost a 750ml ready to use micelles based bathroom cleaner works out at £0.50p
Hope this is of benefit and if I can be of any further assistance please just let me know