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Especially the freeholder is an ass. My landlady had to pay for the cost of replacing the doors after the break in because of issues with the building insurance, that's cost her £3000 (emergency locksmith and next day carpenter don't come cheap). I am not hopeful the freeholder will agree to it... it has something to do with the structure of the building therefore the freeholder will have to agree, right?
As for the size, my front door is a little narrower than standard, so it would probably be more expensive as things would have to be custom made...
You should be absolutely fine for <£1000 all in. Not really my remit but we've secured some of our property at work using grilles/gates and the costs always seem pretty reasonable, I'd assume domestic costs might be even lower given you're likely to be talking about standard sizes. A quick look reveals you can buy the standard gates on ebay for <£200, a decent handyman should be able to take care of the installation.
It's reasonably invasive though, biggest difficulty might be getting the landlady to agree to installation rather than agreeing to meet the costs. I had exactly the same feeling myself after getting broken into in a rented place, desperately wanted to increase the security but ultimately it was all out of my hands. Very frustrating. Good luck!