1.Should I have been made aware of this prior to renting or is this my responsibility to investigate?
There was probably not a legal duty to let you know, and in the absence of a legal duty, caveat emptor applies and it is your responsibility to investigate. Normally there is a duty to disclose defects in the physical condition of the property but not a duty to disclose things that happened at that location in the past.
2.Will I be unable to claim my insurance if my flat is broken into since I told the insurance company there are two locked doors between
the outside world and my belongings (when in actual fact it's one
poorly locked door)?
You didn't commit insurance fraud when you told them something that you believed in good faith to be true. But, now, having learned that your previous statements to the insurance company were inaccurate, you probably have a duty to correct your inaccurate statements to them within a reasonable time.
Whether or not a statement that is not corrected within a reasonable time would invalidate your insurance when the time came to make a claim would probably depend to a great degree on whether the omitted information was material to the incident that gave rise to the claim.
If a burglar came in through the window, or a fire in your kitchen burned down your flat, your failure to correct your insurance application would probably not bar your insurance claim. But, if a burglar broke through your front door, the insurance company might very well be within its rights to deny your claim on the grounds that you did not correct your insurance application within a reasonable time.