I am not a boiler engineer - I just guess based on inferior knowledge.
If your boiler actually started but then failed to continue it might be a fan/pressure switch issue but that's just wild guessing - I have no idea what the roofers might have done.
There's an interesting article about an intermittent boiler saga here... Profile 50e
http://kaijaks.co.uk/blog/nick/2008/02/10/dodgy-old-boiler
More Good Luck..., February 2016