A copy of #25, effectively the first issue of this series, sold on eBay UK in Aug 2010 for an amazing £362.17. It was a VG maybe VG+ copy with tape on the spine. This is one rare comic! The vendor said he hadn't seen one for 40 years!
Valued contributor Jonathan Larsen tells me that this comic was subsequently graded as a 5.0 Very Good/Fine CGC after some professional restoration. Shame it was restored - leave things alone!
There is also a facsimile edition printed in the 1990s on better quality paper which is scarce and sells for around £10 in nice shape. The original issue is more likely to be lower grade, lesser quality paper, probably worn, torn and taped so there shouldn't be much confusion mixing the two up!
Aug 2014: what is thought to be the only remaining copy of #26 fetched an incredible £2605.55 on eBay. Is it 'worth' that much? Clearly this is a classic example of a comic only being worth what someone is prepared to pay for it. Probably graded around a VG+ 4.5.
Note: some PR issues may have no value. Selected issues in FN, VF or VF/NM may not exist in some titles.