The Parkin supplement was never £25.00 from the HMRS - perhaps you bought your copy from Robert Humm
I hesitate to flag this up, but
all Ian Allan books are available at significantly reduced prices from Amazon, this is because Ian Allan have decided to give preferential terms to Amazon so that they (Amazon) can sell in a "Every little bit helps" way to their customers.
All of which rather sours the relationship between Ian Allan and retailers operating outside of the "rain forest". It also means that I stock the minimum of Ian Allan stuff "in da shop" that I can get away with.
On "small ticket" items this perhaps doesn't really matter but on a book retailing at £40.00 plus I don't blame customers for wanting to save what is a significant amount of money. Of course Amazon can get things wrong and is generally a pretty lousy source of bibliograpic information - but then of course it's brilliant in many other ways.
I do wonder whether Ian Allan isn't inflating RRP so that it makes sensible money on its Amazon sales, but if this is true it's rather taking the proverbial out of the "non rain forest" retailing world.
Either way I think giving huge discounts to the likes of Amazon on what are specialist publications is at best short sighted and at worst commercially suicidal.
Just my opinion of course.
Simon