Here is what I found on the www.ico.gov.uk website.  As a result, we will need to consider more carefully how we go about our viral marketing in future. I've pulled out the bits that are directly relevant to my original question and our experience of viral marketing.

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So-called "viral marketing" is where

a) a marketer asks a person to send the original marketing message to a friend or friends, or
It has come to our attention that some companies mistakenly see either of these options as a way of getting around the prior consent rule.

We recognise that your customer might recommend a good deal to a friend whether you prompt them to or not.

This is the sort of thing that individuals would do acting in good faith and in the interests of their friends.

Arguably, where a) applies, you are encouraging one of your customers to break the law (that is, send an unsolicited message to an individual subscriber without prior consent) in order to promote your name. Clearly, this would be a bad way to promote your name and your products and services and you are strongly advised to tell your customers only to forward emails to those they are certain are happy to receive them.

We would point out that where you incentivise them to do so, there is a strong argument that you are the “instigator” of the message. They wouldn’t do it without the promise of a reward from you. You are reminded that it is the instigator of the message who is liable for the sending of that message. A person who allows their line to be used to break the law (i.e., your customer who is passing your message on) may also be liable in this scenario. 

To read the full section on viral marketing download the guidance notes part 1 
http://www.ico.gov.uk/eventual.aspx?id=96

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Hans de Kretser
http://www.dekretser.com