for best flavour, defrost and then roast.
Roast for 30 minutes per lb (450 g) @190 (LB's temps and times are fine too) and make sure you baste the lamb at least 3 times while it is cooking. If you like to serve your lamb quite pink, (which I do), give it 30 minutes less cooking time. Personally,I would cook it for 90 mins and give it at least 30 minutes resting below.
To tell if the lamb is cooked to your liking, insert a skewer into the centre, remove it, then press the flat of the skewer against the meat: as the juice runs out, you will see to what degree the meat is cooked – the pinker the juice, the rarer the meat.
Don't forget to cover with some alu foil and a tea towel on removing and let it rest for 20 to 40 minutes so that the cooking juices in the meat balance through - you should not have them running out on to the carving plate as much.
The biggest error people make in roastingis to over-roast and not let the meat rest (any meat benefits).