Manufacture date of Hotpoint WMA33N, s/n 78117343?

I have a Hotpoint WMA33N, serial number 78117343. How can I discover the manufacture date of this machine and thus prove it must still be within its 5 year parts warranty? (Receipt has been lost).

(It has noisy bearings, I phoned the Service Centre and they are charging £94 labour, plus they won't cover the cost of the parts unless I can produce a receipt to personally show the engineer the machine is less than 5 years old. As we bought it in Jan 2002, I don't see how Hotpoint could even have been making them 5 years ago - otherwise it was hanging around in someone's warehouse for at least 21 months???)