I have been stupid.
Yesterday I ran Week 9 Run 2 and struggled - and ended up with numb arms. I posted here and people were rightly concerned and suggested I stop and have a check up. I was so miserable and worried that I deleted the post. I looked at some websites which recommended shaking your arms and wiggling your fingers to keep circulation to your hands.
Today I decided to go for Week 9 run 3. I'm sorry for ignoring your advice - and I'll delete this post if it is a poor example.
So I set off at a gentle pace 7.5 kph on a treadmill - waving my arms about every 2 mins and wiggling my fingers. Got to 30 mins feeling fine. Kept going. Felt fine at 40 mins but decided to stop there.
By my maths that is 5K. I finished C25K with a 5K run. Slow - and against good advice.
Thanks for all you comments and sorry for ignoring the advice. I promise to be careful, but I don't want to give up running.