Any thoughts on what this is?

aoleks

get a proper heavy duty drill, the longest concrete drill bit you've seen and drill a small hole in the middle, perpendicular to the base. that will shed some light (literally) on what it is. I also advise you to turf around it, get creative, turn it into a feature.

bakepot

stuart45 said:

Are you sure it's an out of use drain? The poly might be there to keep the concrete out of the chamber.

I'm not sure at all it's a drain. Just guess. I see what you're saying about the poly. 

stuart45

It's worth finding out if that's a brick inspection chamber for drains that are in use. If so, it would be better to remove the concrete and replace with a frame and cover for access. It's surprising how many people bury them.

Mistral001

It looks like an old foundation for something as the sides seem to slope inwards as you go down. A hastily dug hole will typically have sloping sides.  There are no signs of bolt holes on the top, but these might have been plastered over when whatever was on top was removed.

dgp1000

Have you looked at the plans and searches with your deeds? These might indicate a previous use of the site. 

Sarahspangles

If there was previously a house on the site and the brickwork you can see is of fairly good workmanship then it may be an outside toilet/washhouse and possibly even a brew house.  If this connected into a tank or the foul sewer was broken then capping off was probably the easiest option.

Looks like you’re going to be shopping for a bird bath.

bakepot

Well after chipping away at it with a hammer and bolster I've discovered the centre was a bit softer and there appears to be the remnant of a metal pole which I'm guessing was a clothes dryer. So 10 points to whoever guessed that. I'm not convinced that's its only purpose but I'm now tempted to buy a sledge hammer and just smash it to pieces to find out

bakepot

update pics

greyteam1959

It could be a cap on a soft water tank.
I have one in my back yard.

Mistral001

It is a foundation for a rotary clothes line.