>No difference Viv - v strange

Seems odd. GroupBy() shouldn't disrupt to previous OrderBy. You could try:

var results = (Context.invoices // Start at invoices because this is the list we really care about
                    .Include(inv => inv.client) // Eager load the client so it doesn't require additional queries
                    .Where(inv => inv.invoiceamount >= InvoiceAmount)
                    .OrderBy(x=x.cientid)
                    .ThenByDescending(x => x.invoiceamount)// Filter results
                    .GroupBy(inv => inv.client) // Group results by client so we can loop through the clients
                    .ToList()); // Execute the query

If that doesn't work I'm running out of ideas :-}
Unless you can use an anonymous type ?