I should have read the pattern carefully - it clearly states:
4ply yarn: 3 x 25g ballsThat means I needed at least 51g, possibly as much as 75 grams to complete the job. Why did I not see that? My only excuse is the earliness of the hour - I am not at my best at 6.00 on a Saturday morning!
So, after knitting for a few more hours over three days, I frogged back to three centimetres and started again!
Lesson learnt: read the pattern carefully!
the silver lining: I taught myself how to insert a needle into the row I want to frog back to! The only thing is, I'm left-handed so I work from left to right which means my row was on the needle backwards! :-)