Thank you all for your comments. In answer to the various questions, the bedding is 100% cotton. I don't think I've been using too much washing powder, and rinse with vinegar rather than fabric softener, as it's supposed to get rid of more of the detergent.
I haven't tried Glo White, but may well give it a go.
I used to do the laundry on 90 degree wash, but stopped because I was worried about not being eco friendly.
Not sure about peeing on the sheets, but I'm sure my husband would be happy to give it a go! lol.
Not much sunshine in Wales at this time of year.
Don't really want to replace with a different colour, as people like to see that the bed linen is REALLY clean, with no suspicious stains that might be hidden on coloured sheets.
Have now tried Googling the Reckitt's Blue, and found some instructions, so for anyone who's interested, it can be added to water and then put in the final rinse of the machine, but that sounds a bit iffy to me, as some people have said they've had patchy results, so I think I'll go back to using the hottest wash, then try Glo White, and if that doesn't work, will get the Reckitt's Blue, and try doing the rinse the old fashioned way, and put it in the bath for the final rinse so that everything gets thoroughly covered.