Qlex,
I'd forgotten all about my postings regarding .htaccess
In the end, I solved my problem with a completely different solution. Instead of trying to redirect using .htaccess, I found a domain registrar with an add on product called "Domain Manager". I transferred the customers domain to this registrar, and was able to perform the redirect at a domain level instead of a website level using the extra functionality. Customer also had to get something done on the target site as well. What this was I don't know. Perhaps this may also be an alternative for you. At least this post now has closure.
Big thanks to Richard for all his help with the code.
Lexy