Ever considered it's in Manchester so that disgruntled Lewisham residents don't turn up in person? (Well, at least not too often....)

p.s. Lambeth's accounts payable centre is in Leicester - they scan all the invoices and enter them onto a database and it's all electronic thereonin... Woe betide you if you don't include the purchase order number. I despair at the number of times invoices just seem to disappear between London and Leicester.