Yes massive light years because they're not going to get a judge to sign a warrant and they're not going to be able to enter without one (according to that) unless a crime has been committed. [] Whilst the OP is claiming they're smoking cannabis, the police would be in a lot of trouble if they tried to force entry without any evidence and also for such a small crime, that they normally just hand out the odd street caution for. [] But yes technically at a huge stretch I suppose they could try to force entry because Bob and Barbara are having a puff
The police don't need a warrant if they "have reasonable suspicion" a crime is being committed. Which (as I said) covers possession of cannabis. Which is why "I smelled cannabis" is a very popular reason for police to force entry or a search (as recent events with police stopping cars shows).
They also don't need a warrant if you are arrested on the street - they get a free pass to undertake a search of any premises you were seenleaving and any premises you give as your address.
Once entry is established, it matters not a jot if the suspicion that gained entry is confirmed or not. The police are in, and that is that. The UK doesn't really do "unlawful evidence" so bad luck.
Since we know the neighbours have a child, there's a whole raft of child protection laws that could also be just to justify a forced entry.
Over the years I have known a lot of people that have found themselves making tea for the police without a warrant and without giving them permission to enter - it happens a lot.
If you're a regular user, cannabis stays in the blood and hair for a long time, long after its effects on you are worn off.
If people are worried about that then it's far better to take cocaine or heroin. They are excreted pretty quickly and rarely picked up when people are stopped driving. It's why they are the drugs of choice in prisons which run testing regimes. Much less chance of being caught.