You had a 25 year lease. You handed the property back to the landlord, but did they formally accept a surrender of your lease? If not, then I think that you are still the lessee and are still liable for rent etc. It may be that the landlord has left it like that & will only change it if/when they get a new lessee. That way, they can still come after you for the outstanding ongoing rent (i.e. until a new lessee goes in).
I don't really understand why N Rock want to place a restriction on your interest, but the effect would be that they would have to be involved if/when your interest (i.e. your lease) is terminated. From their point of view the only reason I can think of for them doing this is that they might think you will be able to assign the lease to another lessee for a premium (i.e. get some money for it yourselves). They would then be able to refuse to agree to the assignment unless you used the money to pay your debt to them. However, if there is no possibility of this happening their seem to be wasting their time doing it.
I suggest you make sure the landlord is aware of what N Rock are doing so they can take some action on it if they wish.
All the above assumes the landlord has not accepted a surrender of your lease. If he has, then you have no interest at all & it is up to the landlord to sort out - they should have altered the land registry already.
Incidentally, have you considered bankruptcy as an alternative to debt management or an IVA? In some circumstances it can be quicker and more clear cut.