So I repated the whole process again, to see if I missed something:
1. Deleted SSL cert, disabled SSL for site
2. Waited for few minutes, checked site is ok, and SSL is disabled
3. Enabled SSL, waited
4. Filled in form on SSL tab of website, selected 'Create Certificate', saved, and waited
5. Checked SSL was working with self signed, yes, working OK in web browser
6. Copied 'SSL Request' from ISPConfig into Trustico, to replace the old CSR
7. Trustico generated a new SSL Certificate
8. Pasted new SSL cert into 'SSL Certificate' field for website, selected 'Save Certificate', saved, waited
9. Apache stopped, could not access ISPConfig.
Same error as before:
[Tue Jul 26 22:54:05 2011] [error] Unable to configure RSA server private key
[Tue Jul 26 22:54:05 2011] [error] SSL Library Error: 185073780 error:0B080074:x509 certificate routines:X509_check_private_key:key values mismatch
I'm now going to try bypass the ISPConfig SSL process, and manually create my own SSL key and CSR, see if that works.