I attempted some plastering for the first time yesterday. I used
Wickes patching plaster on quite a small area 0.3m^2 -ish. I am
pretty sure the mix was a bit too wet. Anyway, this morning, there
were yellow stains in the plaster. Is this likely to be sulphur and,
more importantly, is it a problem? Should I hack it off and try
again?

It's a 30s house, the patch had the original wooden battens as a base.

Thanks.