I posted in the children's health section but was advise I might get a better response here.
The rash is on her back/ neck. No temperature or other issues. I've been treating it with diprobase cream and piriton for the itch. It started 2 nights ago and is fading a bit now.
I also cancelled her swimming lesson yesterday as I didn't think chlorine would be the best idea.
The only think I can think could have caused it would be the jelly we bought (hartleys glitter jelly) as all other food she's had loads of times with no problems. We haven't changed washing powder but she did briefly try in a t shirt I bought from the charity shop to see if it fit. I suppose she could have been allergic to whatever that was washed in but she only had it on for less than a minute so not sure If that would have been long enough to case a rash.
She did have an allergy type rash a few times previously on her chest. One of time it was caused by sudocrem and I couldn't figure out what caused the other reactions. I did suspect Ariel liqui tabs one of the times so stopped using them on her clothes. I also tend to use the allergy rinse cycle on my washing machine all the time as I sometimes get a reaction if clothes haven't been rinsed enough.