According to one UK dealer the 1997 Castle High values, designs from photos by Prince Andew, set of 4 values - SG 1993/96 (Enschedé Printing) is believed to be "the scarced set of of all the George VI and Queen Elizabeth high values "- presumed "to be short lived with minimal usage".
"Issued in 1997 and suddenly replaced early in 1999 with Mini Machins - most Post Offices were still using the previous Harrison Printings, so very few fine used copies, especially the £1.50 value (see photo above) came into circulation".
Similar to the Harrison Printings SG 1612 -1614 these have slight differences to the design - namely The Harrison stamps in the word "Castle" the "C " has a top serif and the tail of the letter points to the right, the "A" has a flat top and "S" has top & bottom serifs.
Enschedé: Re-engraved. The "C has no top serif and tail of the letter points upwards, "A" has a pointed top, "S" has no serifs.
The Queens head ( said to be a Machin design) is in optically variable ink which changes colour from gold to green when viewed from different angles. This is printed by silk screen process in silhouette. The Enschedé stamps also have one elliptical hole on each vertical side of the perforations (15 x 14)
I have not checked the price in recent catalogues but the 2004 Concise prices are as follows:
£1.50 = £07.50 mint, £4.00 fine used
£2.00 = £08.50 mint, £2.25 fine used
£3.00 = £25.00 mint, £3.30 fine used
£5.00 = £25.00 mint, £10.00 fine used