Plaster Cracks Suddenly Appeared in 1980's Semi Detached

[brutes]

I have recently noticed plaster cracks on internal walls in my 1980s semi d. They literally just appeared one day . horizontal , on about 8 walls internally and approx 1 metre in length. not deep. plaster bubbling is what they look like and when i peel off some I can dig in a small bit less than 1mm I reckon. They look very bad/worrying.
House is set off a slightly sloping driveway approx 1 metre below street . gutters leaked last winter and drains also were clogged leading to build up of water of c 2/3 inches at times in heavy rains .
No wetness or flooding internally. House is a bit damp feel wise

Any ideas or should I be seeking an engineer to inspect ? Or just replaster/repaint ?

[welsh brickie]

normally most cracks are superficial, but I would have a structural engineer view the cracks, for piece of mind