I have just upgraded from 1.3.9. to 1.5.5.
Thought everything was OK but have now discovered that something is out of sequence.

If I place an order as a customer, the system registers the order no (say 8002). If I look in 'Customers Orders' in admin I can see the order as 8002.
Paypal is happy to accept payment and the system then returns to the checkout success page
HOWEVER, the 'Checkout success' page - Thank you for your order etc - gives the order information for the previous order placed (by me) ie 8001.

If I use PHP myAdmin to examine the database records then ...
the last record in 'orders' file shows 8002.
the last record in orders_poducts shows 8002
the last record in orders_products_attributes shows 8002
THe email archive shows 8002 but I have also noted that whilst (Email options 'Send copy of confirmations' is set to send a copy of the order to me and the office) it sends successfully to the office, it doesn;t send an email to me or the customer (in this case again me)

I presume that the incorrect number is being held somewhere else but can't think of where to look next.
Anyone got any suggestions please ?