Hello Ian,
Judging by the serial number I would say your machine is about a 1990. Olympia closed up in 93 and was reopened in 96. My serial number begins with an 84. My understanding means it was made in 1984. I don't know this for an absolute fact, but I am pretty sure. As to value, that is difficult but I will tell you what I know. You can view pictures of my Cremina on my post--Using the Olympia Cremina. I paid $660 on ebay. It was in perfect working condition with no blemishes sans one small scratch. It did not come with the knock box, dosing spoon, tamper, pitcher, or double basket that comprised the standard accesory kit. I paid an aditional $25 for the double basket.
Roughly comparable machines go on Ebay from $500 to $700. Ones that are rough or need work go from $150 to $350. I believe that Steve paid about $406--before being refunded for shipping damage-- for the cremina he is rebuilding in his "Restoration of an Olympia Cremina" post. I saw one in rough condition missing the lever, portafilter, and all of the accessories go for $150. There is a really nice one on ebay right now with all of the accessories with one day left you can watch to see how much it goes for.
http://cgi.ebay.com/ws/eBayISAPI....em&item=4397125581
Being that it is in good cosmetic condition but needs a gasket kit, I would place the value between the $660 I paid and the $406 that steve paid. The gasket kit costs about $90 and a couple of hours of your time. Add all of this into the equation with how much you can afford and how little he is willing to sell it for and you should find your price. I hope this helps.
Lance
