I was trying the Timer fuction this morning but had to go out and didn't try selecting the type to once. I'll have to wait till later this evening before I can try again!
All Im doing is making a boolean true then false on a set of sprites while toggling the visability of another sprite on and off respectively.
...Having just written that out I think it's because visability of selected sprites are getting called at the same time... I know what I'm doing wrong now