Dave,
Given a choice between believing any politician and trusting my own judgement I would chose the latter.

Then you are a fool. What knowledge is your judgement based on?
Have you studied the reports available to governments?
What do you know about any of this Dave apart from what you read in the papers?
Did you take into account that he has previously murdered here, and lied about that as well as lying about cheating in the Olympics, lied about its intervention in Crimea, lied about the shooting down of a Malaysian airliner, lied about their murders of Litvinenko, liberal politician Boris Nemtsov and investigative journalist Anna Politkovskaya,...

These governments have seen the actual evidence and based on that evidence their judgement is that Putin and his regime lied about this too:-
United States: 60 United Kingdom: 23 Ukraine: 13 Canada: 4 France: 4 Germany: 4 Poland: 4 Lithuania: 3 Czech Republic: 3 Spain: 2 Australia: 2 Denmark: 2 Italy: 2 Netherlands: 2 Albania: 2 Estonia: 1 Romania: 1 Sweden: 1 Croatia: 1 Finland: 1 Latvia: 1 Hungary: 1 Macedonia: 1 Norway: 1