Water constantly running into basin from the cistern, Simple Repair.
The unit fitted to this toilet is an ideal standard duel flush.
The Rubber seal on the bottom of the cistern has perished, it can be replaced for less than £4
Link to New Seal on Amazon, https://amzn.to/3bmuo0J
There are 2 sizes available, from ideal standard a long one like in the video, & a short one,
Link to New complete cistern (long) £28. https://amzn.to/2vGGJxz
Link to New complete cistern (short) £26. https://amzn.to/2vDdHyG

Link to all tools used, https://www.amazon.co.uk/shop/aaanton11