B331-1963 GB Queen Elizabeth II “Red Cross Centenary Congress” 3d purple block

Title:
1963 GB Queen Elizabeth II “Red Cross Centenary Congress” 3d purple block of 4 – Mint (UNMOUNTED) SG 642 / SG 642p
Category:
Collectables → Stamps → Great Britain → Queen Elizabeth II Commemorative
Condition:
Un-mounted mint block of 4 (corner margin, full gum) – minor hinge remnant/tape visible on reverse (see photo) so grade as “Unused (some hinge/tape presence)”.
Item Specifics:
Year of Issue: 1963
Country/Region of Manufacture: United Kingdom
Denomination: 3d
Catalogue Number: SG 642 (ordinary issue) / SG 642p (phosphor band)
Format: Block of four
Mint/Used: Mint / Unmounted (with some minor gum issue)
Theme: Red Cross / Charity
Valuation / Price Recommendation:
Catalogue references show the ordinary set SG 642-644 for GB listed at modest values for fine mint (ca £2-£3) and for phosphor band varieties higher (for example SG 644p ~ £27.99 fine mint) at specialist dealer pricing. 
On eBay recent comparable block listings (MNH) show around US$20-30 (~£15-25) for common mint blocks. 
Considering yours is a block of four (which is more desirable than singles) but has a tape/hanging-hinge remnant on reverse which lowers the grade, a realistic valuation might be in the £20-30 range.
Recommended eBay Listing Price:
Set a Buy It Now price of £29.95 to allow some room for negotiation, with “Best Offer” enabled.
Auction Option:
Start the auction at £9.99 with a 7-day duration. Use a reserve price of around £18 if you want to protect the value.