I'm certain there is a product similar to a silicone that can be injected into the cavity and then it sets hard. However, I'd still get a trustful builder, who will make certain that the lintel will remain strong and the crack hasn't appeared due to excess weight of the building above it. Hope that helps. Good luck with the renovations. Denise

When it comes to Asbestos it's always advisable to err on the side of caution and avoid any risks. Take a look at this government website http://www.hse.gov.uk/asbestos/gallery.htm and I would suggest sending your images to a licensed contractor for an initial opinion and they will be able to advise and arrange a visit if necessary. Don't touch it or continue with any works in the meantime.

Asbestos is very dangerous, you should get a professional in to carry out checks to determine if it is indeed Asbestos. You certainly should not continue removal of the tiles from the other room. I read somewhere that it takes at least 72 hours for Asbestos dust to settle so having children in the house whilst you were doing this is not good even if it isn't Asbestos dust. Research reveals that long term exposure can lead to lung disease and cancer with symptoms showing after 20 -30 years (I do hope it isn't Asbestos) One more thing i'd do is not use your hoover as if it is Asbestos, you could be re-introducing Asbestos fibres around your home.

Jali is an online company that’s been going for years - they do various made to measure covers. https://www.jali.co.uk/product/radiator-covers Radiator covers aren’t always cheap (unless you find a fab carpenter as Daisy suggested) - so another option would be to replace the rads with traditional column versions - you can get some amazing ones!!