Never believe in stamps alone!
Every stamp should have a printed, detailed receipt with the breakdown of work supporting it.
On another forum I sold a car with no history. The buyer then tarted up what was a good car, but falsified the history with additional stamps in the book and mickey mouse receipts produced on his computer. The stamp was a main dealer Fiat stamp, but the receipts weren't of course.
He had it on ebay and Pistonheads, he got reported! The ad vanished but no doubt he sold it one way or the other.
Service history IS important but I agree, not so crucial on a car this age tbh. A car may have been specialist or main dealer serviced all it's life, then thrashed to death from cold every day between services! I gauge cars on condition, history if it's there, and receipts, the seller is also a good indication, a bonus if that owner has had it a while too as if you think he's abused it, you know it's been abused for some time!
Anyway, I'm moving off track! Considering the condition if it's correct, the rarity of the (marmite) colour etc, I reckon your close to the real value with the belt and history issue in mind.
Best of luck with the sale.