Checkout success page shows previous order no

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 ?

What you've described is a classic symptom of using PayPal Standard, where payments are NOT stored in real time, but instead are dependent on waiting for PayPal to notify your store in the background with an IPN "Instant payment notification" message sent to your your_domain.com/ipn_main_handler.php ... and unless that completes successfully the store won't record the order at all.

Using PayPal Express Checkout gives you the benefit of immediate real-time confirmation of payments so you wouldn't have these symptoms.

If you're really bent on use the old "PayPal Standard" approach, then you might be able to troubleshoot your problems following the comments I posted here: https://www.zen-cart.com/showthread.php?61199-Debug-Logging-PayPal

Ah ! Many thanks for your prompt reply. I've moved over to Paypal Express and will see how that goes - certainly looks to have fixed it. The slight red herring of 'missing emails' was down to googlemail deciding that my shoppping cart emails were now 'spam' Also fixed.