It always amuses me that Stanley Gibbons speaks of market prices. They tend to be the most expensive source. Other dealers then offer their items at a discount to that price. How that could be called a "market price" remains a mystery to me. Only some opportunists on eBay
try to get a higher price.
Even at their own auctions, their estimates are far below their catalogue prices. If they would not, you would be better off buying from their stock and avoid the buyer premium. It raises the question what market sets the prices in their catalogues. Although I found that some high-quality classic material is now going above full SG-catalogue price.
Of course, they mostly have high-quality material. Often, you will not be able to buy items prices up to £50 in their catalogues from them, because they seem to have focused on premium items and sell basic sets.
My 2005 SG specialised lists all but the ½d and 2/6 values from this set with a sideways inverted watermark. All are priced in mint condition only. Most carry only a small premium to the basic stamp. Considering postage dues were affixed to items by the post office, I doubt they are common in mint condition but not in used condition.
I noticed most postage dues have considerably increased in price even since 2005. Interestingly, the mint prices in the 2005 SG specialised are up by £35 for the sideways watermark, unchanged for the sideways inverted watermark, both in unmounted mint condition, but down to £10 for the "normal" stamp in used condition.