Just to update you Viv your anonymous type code worked perfectly as well - thanks again.

Edit

This is what I've settled on

var results = Context.invoices.Where(x => x.invoiceamount >= InvoiceAmount).GroupBy(x => x.client)
                .Select(group => new { companyname = group.Key.companyname,Id = group.Key.id, invoices = group.OrderByDescending(x => x.invoiceamount) })
             .OrderByDescending(group => group.invoices.Sum(x => x.invoiceamount)).ToList();