the mistery is resolved - but remains a mistery
I switched off apache modules one by one to see what caused the issue.
PHP was fine it seems, so in my view the failed Imagick installation from source did not impact the server.
SSL finally prooved to be the problem. Apache started fine without that module. Out of curiosity I restarted SMTP and POP3 - they also failed to restart - I believe they also use SSL libraries. Interesting. This would explain (although not very satisfactorily) why Apache did not report a problem - the problem was within SSL itself.
So I figured it was the new SSL key or the new IP address. I reissued an new key for the certificate, enteredit into ISPconfig and lo - it works again.
I find this really bemusing. A signed SSL certificate (maybe faulty? maybe not?, who knows, but certainly from a reputable vendor) brings down apache so it fails on restart without error. I would not have htought this was possible. One would have thought exceptions like thesee are cought, rather than bring down all services using SSL.
Anyway - we live and learn.
Thanks for everyone who read the post - although I had no response. I hope this is going to help someone else in the future.