O/T: Recommend a bug sweeper?

Serious question. I run a very small intellectual property law firm. A number of clients have asked how secure we are in terms of corporate/industrial espionage. Unfortunately we are unable to assure them that our premises are bug free.

Frankly I think the probability of having bugs in my premises is about zero but to satisfy my clients that I have taken due diligence in this regard, can someone recommend an inexpensive but reliable bug detector?

I've looked around personally and they can be anything from a few quid to thousands of pounds. I don't want to look like an idiot holding a box with flashing LEDs that does just that, but I also don't want to fork out too much money over what I think to be a theoretical risk. My budget is at most a few hundred quid, but I'd rather spend under £100 for one.

Purchasing an inexpensive bug detector, even if such a thing exists, doesn't sound like due diligence to me. You need to go to professional security firms who can provide sound advice, effective help and well-documented evidence that you have taken reasonable steps to ensure that your premises are secure.