Thanks to those who tried to help, including Andrew from PG.

I have solved my problem – sort of.

After loading an all-midi song that won’t play I simply add any RT to a spare track – yes any old RT will do into any spare track.. Now the song will play as normal.
Then I mute the RT track using F5 so that it doesn’t screw up my song and bingo – the song still plays normally!!! I then re-save the song with this ghost muted RT.

I have finally learned my lesson. I have every type of style or RT that I could possibly use so 2012 is the last time I will ever purchase a new version – I hesitate to call them upgrades. I haven’t changed my equipment or configuration and nothing like this has happened in the last 20 years. Did you know that build 819 is on the way with 19 more ‘fixes’

Tony