Alan.
Your hole may not be noted on a ordinary map or indeed the deeds or plans of your house but there is a chance that it is located on a Ordinace Servey Map. I doubt having worked Chislehurst Caves for by on 25yaers that it is a Dane hole in fact I rule that out altogether. O.S.M. had to place such hidden chambers if known about on there maps in order has a reference to building ,road making etc etc. Ice Wells are mainly made up of red brick with a spiral staircase down the side and they are usually deep or the ones I(have been down are over 30ft if not more.any shallower and the effectiveness of them has a Ice well would be impaired most Ice wells are deep in order to avoid down drafts from the surface also at a certain depth you can maintain a certain temperture so I rule out a Ice Well.

I would not be surprised once you get rid of the last of the earth that you may? come across another manhole. Clearly you hole has seen little activity has you say there is no names dates or Graffiti(did I spell that right I fear not. It could be a excape shalft from some underground bunker I have seen several. The debris you are clearing could be a vain attempt to hide the lower hatch once the bunker if it is a bunker was abandoned. and the top hatch is usually covered by a layer of Earth and possibly grass seeded to hide the outer iron plated hatch. On most excape shafts exspecally MOD. or army or deep in Deep bomb Magazines there were two hatches to most excape shafts and there would not be any piping or window holes or anything remotely like it . It would appear once found from the surface to look like a doomed chamber.

I fear you back garden could hide an old bomb Magazine or deep underground explosive storage depot. Is there any reference to anything like that in your area Richard Bridge is a learned Man on the area in q00stion. I nput the question to him?
Kind regards Pierre Le Chapeau.