I have a rather simple HTML E mail which works fine;- basically it is all in one table with the table styled to give the right colours and positioning.
However I have a standard web page for my site with drop down menus etc. Is it possible to use this to create an e mail which I could send to clients.
That's unlikely to be something that you can achieve reliably. Although browsers are pretty standard now in terms of rendering HTML and CSS, email clients are a long way apart and in some cases going backwards (not mentioning any names, Outlook, but we're talking to you). Pretty much any code that you need to make an email look good and work effectively beyond using a table with sliced-and-diced images will fail on at least one major email client.
Back to a table with all the CSS Style at the top of the page ?
Actually, quite a few email clients strip out any CSS at the top of the page, so a more reliable solution—horrible though it be—is to use inline styles on everything.
Mind you, perhaps it's best to let crumby email clients fail like this, so that people will stop using them. (I don't think it's going to happen any time soon, though, and if you are doing this for paying clients, they probably won't be too happy to be sending out broken email templates.)