In reply to Dominicandave:
Firstly you would only be entitled to £1 back from your £1 each way bet(£2)as one of those £1s was for the win.
Secondly I've never heard of the fraction 100/3 as odds on a horse or dog as normally it would be 33/1
Thirdly If the cashier has made a mistake and written 100 / 3 instead of 100 / 30 on the slip they will not pay out. They will just claim it was a genuine mistake.
How you / they have got to the figure you've been given god only knows..
cheers
Gaz