The 'mortars' in 1991 were the standard PIRA design (Mk 10 IIRC, but I could well be wrong) which were basically large gas canisters full of home made explosives with a small tube containing propellant welded to one end and set off by a time delay, and launched from a frame rather than a tube (more like a 'Katyusha' rocket in many ways). Horrendously unstable, inaccurate and indiscriminate: the attack on 10 Downing St didn't really 'miss', it was about as pin-point as it was possible to get with them. I was on the wrong end of a couple of such attacks in Northern Ireland in the mid-80s and - both times - the terrorists failed to get them into the relatively large SF base I was in, from a range of 2-300 metres at most, and hit neighbouring housing instead. All in all, very trying but no doubt the reason why Whitehall was cordoned in this instance, and I suspect the miscreant would have been facing a short, sharp interview without coffee from his superiors.