Colin
What you need is PBR (Policy Based Routing) which allows you to specify the next hop or outgoing interface based on the traffic type. Unfortunately the ASA does not support PBR, for this you would need a router running the firewall feature set.
Edit - actually rereading your requirments it may be possible. If you want to use the ADSL for all internet browsing from internal clients then you could set the default-route on the ASA to be the ADSL next-hop.
If all the traffic via the leased line ie. HTTP and SMTP is for incoming traffic ie. you are hosting web servers/mail servers then you can simply present these servers with an IP out of the leased line range if you have spare or the leased line interface IP address on your ASA.
Jon