It is likely you can see erosion of the grout, even cracks in the grout, fix these and you are a long way towards fixing the issue 'temporarily' and the only correct measure to repair this issue is to remove the tiles and provide a waterproof membrane as described above.
Try the instant repair method and see how you go... but be aware that sometimes these 'temporary' measure don't work.
Anything else needs to be discussed via PM as it agitates me too much.
Rather than pay lots of money for someone to come in and waterproof the shower I thought I would do it myself. The product I have been sold by Bunnings is Bondall Tile and grout Sealer (Tileguard).
Can anyone comment on whether or not this is a good product for what I need or is it totally the wrong thing. Just have some doubts as it seemed quite cheap.
Any other advice on this job greatly appreciated.
Regards
DJ