I have just bought a property where this was kind of an issue - the previous owners suspected subsidence 9 years ago, and their building insurance company (the Halifax) monitored the cracks for 2 years, and it turned out there was no movement, so no claim was made. The repairs they did at the time also remained OK, so I wasn't worried when I bought it. The surveyor said it might have been tree root problems from the road where the council had planted them, and subsequently pollarded them. Might be worth chatting to the district surveyor to see if there are any known problems. But it is worrying that the cracks are diagonal - why not call your insurance company and talk to them about getting it monitored?