redirection problems

I'm using .*$ http://www.new-domain.co.uk/ in a .htaccess file to divert any
requests to an old domain to a new domain at the moment because it strips
anything from the original request and redirects to the root of the new
domain (there is no similarity in the structure). However I'd like to
redirect to http://www.new-domain.co.uk/?old-site to track visitors coming
from the old site more easily. When I put that into the .htaccess file it
actually redirects to http://www.new-domain.co.uk/%3fold-site . What should
I be putting in the .htaccess file to get the question mark to come out
properly?

Many thanks,
Geoff

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