Should I be suspicious of rendered walls on a terraced house in the uk?

Many older houses are rendered and at one point it was quite fashionable. Generally render does not cover significant cracking for very long because the render itself will also crack and is in fact more likely to because it does not have the same strength as brickwork or masonry. It was often done because it could then be painted and look a lot better than painted brickwork - my own house has had such treatment and is perfectly structurally sound. It was also frequently done in areas which had high exposure to the weather such as near coasts as it protected the brickwork from deterioration. If there are significant cracks in the walls they will usually show on the inside of the house too. Incidentally, you will do far better to employ an experienced builder to examine the property rather than a highfalutin "qualified surveyor" because they are the guys who really know what's what when it comes to construction. "Surveyors" just know the theory, almost never get their hands dirty and have get out clauses in their contracts.

I would want a full survey from a properly qualified surveyor.

I would be. Get a full survey.