Thanks for all the posts folks. I think the definitive answer is as follows:-

select datediff(dd,start_date, getdate())-(( floor((datepart(dw,start_date)+datediff(dd,start_date ,getdate())) /7 )*1))+ case datepart(dw,getdate()) when 7 then 1 else 0 end

Seems...

Thanks for all your help folks. I'm going to do what I should have done a long time ago and ask for a working days calendar to be added...