I hope a few others will post here - the usual suspects and some of my favourites !
Ground rent (over the last 300 - 400 y) has been subject of various rip-off and you have to know the rules. Once you are up to date, the current rent only is payable altho unscrupulous owners may demand two or three years up front. They may demand but the status is only an 'ask'.
When I bought my own house, my lawyer demanded six years unpaid ground rent from the seller up front, and to my amazement someone popped up a few years later and ddemanded the back rent. So some back rent is payable.
But was mine limited to six years ? or was it just that I was six years in arrears ?
There should be someone who knows the answer.
Oh, and as well as demand a year up front habitually - but I only pay six monthly, he 'forgot' one year of my payments and tried to get me to pay it again.
You have to watch these people.....and when instead I sent a scan of the cheque for THAT year - he didnt even say oops sorry