One of my clients is interested in hosting his website with me (I have a reseller account) but his email is hosted with 1and1.co.uk and he wants to keep it there.

If I point the domain name to the nameservers for my reseller account, I imagine this would stop his email being received in his 1and1 account?

I have a feeling it will be impossible to repoint his domain name whilst keeping his email hosted with 1and1, is this true?

Thanks

I have a feeling it will be impossible to repoint his domain name whilst keeping his email hosted with 1and1, is this true?

Nope

point the hostname 'A' record to the webserver then create mail. as an 'A' record pointed to the 1 and 1 servers, or as an alias (CNAME) pointing to the DNS name for the mail host.
Set the 'MX' records to mail.hostname

Thanks. I spoke to my hosts and they just said they'd need the MX records for 1and1?

I suppose by that they meant that they'd take care of the A records etc.