Not having an IDP won't be a problem - your UK licence will be fine. In over 12 years of driving around Africa, I've never been asked once for anything other than my home-country licence.
How likely are you to get stopped? Almost certainly. Road blocks are commonplace and you will very likely be asked to produce your licence. The other risk you face is being stopped and "fined" for some (usually wholly invented) offence. Best way to lower the risk of this is to observe the traffic rules to the letter, ie do not speed - EVER - and do the other things they require, like have a hi-vis jacket (which must be draped over the driver's seat-back) and two warning triangles. Your hire company should provide these. Even so, they may try it on anyway. Last time it happened to me, my son was allegedly "not wearing his seat belt in the correct way". Whether you then pay is a moral and practical dilemma for you, along with a test of nerve (and also of your bargaining skills)..... in my experience the risk of this is greatest in and around Maputo. The further away from there you are, the lower the risk.
