If you take photos of their chimney, make sure it is only of their chimney, or when this comes out in the wash they can sue for invasion of privacy!
Owners of property do not have the right to prevent someone taking pictures of their property from a public place. If you are actually on private property, of course, the owner may forbid photography or place any conditions they wish on it.
It is illegal to harass somebody, and taking photographs of their house could be part of a 'course of conduct' which could amount to harassment. But this must be a 'course of conduct' - at least twice, therefore, to count under this heading.
It is likely that a court would consider the use of a long lens to take a picture of someone in a private place, such as inside their home, to be an invasion of privacy, infringing article 8 of the European Convention on Human rights. But this is unlikely to be the case for an inanimate object, and certainly not for an inanimate object like a chimney, which is in full public view and has no reasonable expectation of privacy. In general, if you stay away from photographing people (and especially children) you are safe from any accusations of invasion of privacy. Gondolier is quite right to add his warning, though note that it is usually the 'publishing' of such pictures which causes the breach of privacy rather than just the taking of them...
There are various other restrictions on photography in some public places - Royal Parks, London Squares and the London Underground spring to mind, and restrictions may be enforced in some places under various items of Terrorism legislation. If there are certain rare nesting birds on the flue it would be illegal to disturb them under the Wildlife and Countryside Act 1981, and close-up photography might therefore be illegal.
So unless the offending flue is in a prohibited place under the Official Secrets Act 1911 (as amended), a wildlife preserve, or on certain other special buildings such as a court I think you are safe. The only other restrictive circumstances I can imagine are if reproductions of the flue are controlled by copyright, or if the flue has partially-burnt bank-notes coming out of it (photography of bank-notes is specifically forbidden under the Forgery and Counterfeiting Act 1981 (section 18 (1)). Apart from that, you are in the clear....