What a great placename !

First, I guess from your post you've seen this Will, from TNA site:

D2957/306/25 12 April 1879

Contents:
Conveyance
(1) Christopher Joseph THORN of Pontypool, mason
(2) The Wye Valley Railway Company
Moiety of freehold land (32 perches), part of a close called Lumpland in Tidenham. Plan endorsed
Consideration: £25
Witnesses: A. MORGAN, sol. Pontypool; F.J. EVANS his clerk
http://87.248.112.8/search/srpcache?ei=UTF-8&p=tidenham+lumpland&type=61465&...

I suspect these general Tidenham sites are all known to you, but maybe worth followingup offline ?

http://www.tidenhamparishcouncil.co.uk/history.html

http://www.tidenhamhistory.co.uk/index.html

Unfortunately the ever-reliable British History site contains no references to Lumpland or Thorn that I can find:
http://www.british-history.ac.uk/report.aspx?compid=15758&strquery=lumpland

I would have suggested the Old Maps site to study the old OS Maps but you've done that with some success, so....???

Hope this helps a little, I'll try again later when time permits.