I think I badly worded it. I said :- ""However when side lights are on and ignition is switched off the new high level brake lights go out when I press the brake pedal." I should have said high level side lights go out when I press the brake pedal. I think you are correct jbcollier. An alternative route is being found. I did think a diode would be the answer but wouldn't know how to go about it. The lights are certainly working satisfactory for use and I can live with it, but it was just puzzling me.
Thanks all for your replies.