Wrong password — use "Forgot password?" on the sign-in screen. The reset link goes to your inbox; check junk if it's slow.
"No account with that email" — you may have signed up with a different address than the one you're typing. Try the other one before creating a second account, because a new account starts empty and won't have your quotes in it.
It worked yesterday and doesn't today — try the website at gaffermate.co.uk rather than the app, which tells you whether it's the account or the app.
If none of that gets you in, email hello@gaffermate.co.uk from the address you think you signed up with and it'll be sorted from that end.
GafferMate quotes off your own price list. 14 days free, no card.
Start your free 14 days