Font display in eMail body on PC

Hi all

I have just started using 360Works eMail plugin and everything is going well apart from the problem of the font in the body of eMails read on a PC are all different font types and sizes. it looks fine on a Mac

The eMail sent out has a mixture of standard text and text pulled up from table fields. When reading the eMail on a PC each section is in a different typeface or size. I can see that the text set directly in the 360 plugin script is different to the text pulled from the table fields (not what I want) but the pulled in field text also varies in size from field to field even though it is set the same in the original field

How do I set the font in the plugin so that it is uniform across the whole eMail and in the font and size I want?

The plugin value is this:

EmailCreate( Default Values::Default Accounts eMail Address ; Client Details::Accounts eMail ; Default Values::Default Company Name &" Invoice "& Invoices::Invoice No)

and
EmailSetBody( GetAsCSS(
"Dear "& Client Details::Accounts Contact&¶&¶&
"Please find enclosed Invoice "& Invoices::Invoice No&¶&¶&
 Default Values::Default Invoice Line 2 Text&¶&¶&

Default Values::Default Accounts Signature

); "html" ) and
EmailAttachFile( $exportPath ) and
EmailSend and
EmailDisconnect

Thanks for your help