Thanks for your suggestions. All suggestions resulted in me requiring authentication via the old telephone number including setting up the app. I did not try Twitter nor facebook as you suggested, so cannot say this method would work. However, as suggested I persisted and got to speak to someone who handles security issues on paypal and they helped me access my account. The number that I managed to speak to someone (UK) 02039017000, this is the number listed in the contact us as a guest. I had to deal with a long automated service, they will need your old incorrect telephone number and last 4 digits of your payment method to confirm identity, when asked reason for call, I said "locked out of account". Bear in mind this is all automated and after this there was a 15 minute plus wait to speak to a person. There was an option to choose a call back and not hold, I did not try this as with my luck they would call the number I was trying to change. Confusingly, they give you 3 options, press 1 for call back or 2 for other options or [insert long blank where I thought I had been cut off and the 3 option was not listed] afterwards, they repeat choose 1 or 2.... Choose 2, there you get the option to wait on the line. I mentioned I had tried the 08003587911 number as mentioned in my above post, she said that they had changed the customer service num[Removed. Phone #s not permitted]y this should get you directly to customer services without the wait. Perhaps if someone else uses this method they could mention here if it was easier.
Hopefully how I managed to get my number changed helps someone else.
