Mk1 golf cuts out when I press the horn

I have a 1983 golf gti, recently changed the speedo binnacle and now the car cuts out if I press the horn while driving but not when stationary.

any suggestions, I'm not very good on good on  electrics but any advice would be greatfully received. 

Sounds like you have a flaky Ground, and your Speedo Cable is acting as the Ground point, which isn't good when you go to start the car. High Current Draw through the Cluster via the Speedo cable can result in the cluster Melting.

I would clean or replace the Battery to Frame and Frame to engine Ground wire, then verify that your speedo cluster (clocks as you call them) has a good ground to the Frame on the Brown wire.

That would make sense as the car is now a pig to start.
Thank you so much, I let you know how I get on.

Andy