I don't like leaving negative reviews, but I feel it's my responsibility in this case.
We used Centrona in the past to repair our boiler and then we signed up for their boiler cover which we had for 20 months. The engineer who came to fix the issue twice (once successfully before we signed up for the cover, once not - after we signed up) was our only positive experience. He was very kind and professional.
However, the rest was a disaster. We never knew what the cover included and the only time we had to use it - the issue was not covered. We never had access to the T&Cs of this cover, which is also my fault that I didn't ask for this when I signed up, but just be aware they never send you an email confirmation or a hard copy what's included. We still asked them for a quote for a boiler replacement which I had to chase twice over the phone and they arrogantly told me they're too busy and don't know when it will be sent. This happens in Jan/Feb time, when they're aware their customer (we) doesn't have heating in the house and we're patiently waiting for a quote, potentially paying thousands for this job. When the quote was sent (3 days later), it was missing major details which we asked for + it was only for a cheap boiler brand which we never asked for.
Anyways, it was obvious that there's a massive lack of professionalism and basic marketing/customer support efforts, so we decided to fix the issue with a local heating specialist and just cancel our cover with Centrona.
I called to cancel the cover, which I had to chase twice. The lady on the phone could not do it for some reason, I waited for someone to contact me. When they did, they told us it's one month notice. Just for the record - they never asked why I cancel, why I'm not a happy customer given that we've spent almost £1000 with them in the past 2 years. So this one month notice really annoyed us, given the fact that we were never informed about it (not over the phone or in an email). The agent I spoke to handled it very poorly, making remarks that "they didn't invent the 1-month cancellation" just for me, that he will send me the info even in a "telegram" if I wish to etc. He didn't stop to listen what I was trying to say which was simply - I never knew what are the terms of this cover. He said - "you would have known if you had asked for..." and "how do you know the terms of your Virgin Media account"... Well, I get them in an email and in my account! Basically, some of his comments left me speechless and overall it was very disrespectful. This went on and on, so in the end I just gave up as it was clear there's no basic effort to understand why we were unhappy, let alone to keep us as a customer... We spend over £400 on a boiler cover we never used, isn't a common sense to investigate why we're not happy to pay another monthly instalment, instead of attacking me in such a bad manner...
So in a nutshell, very poor experience on so many levels, so you'll be better off if you find a trusted local engineer who actually cares if you're served properly and you're a happy customer!