Thanks all, hygrometer on e-bay is around 70 notes with delivery, sounds a lot, although cheaper than F. Ball and the likes who want 120+!! Pity these things aren't hireable for say £20, as its a true single use item for a self-builder, and likely to get used once and slung in top of cupboard for years. I have one of the 2 steel points builders moisture meters which give an indication of the level, but guess the hygrometer is only way to be sure?