Hey Paul. Don't get too worked up over it.
I just completed my online test as well. I think it went better than when I did it last year which isn't saying much but I still got through. I struggled with the mental arithmatic as well. At some points I was just staring at the numbers before I realised I was meant to be doing something with them. Last year I swear I missed several questions in a row at some point but still got through stage 1.
Good luck. Stage 1 now booked for two weeks times. Best get my head back into that paper again.