I would have thought that 10W30 was a bit thin for and engine that is almost 20 years old; I would suggest 10W40 or 10W50 - have a look at Oilman's (guy at Opie Oils) thread for recommendations. I wouldn't go as low as 5W oils in the UK, it just doesn't get cold enough...
Also, if you don't want to fork the extra money out; consider normal plugs rather than iridium. - The iridium tip does have advantages, but it is very fine so it fouls easily and needs to be cleaned carefully (if you take them out, but don't replace them). Something like an NGK BCR8ES would probably do just as well, and you can buy a box of 10 for the same cost as 4 iridium ones.
I should add a disclaimer that I'm no expert; don't take my word as the absolute truth. There are people on here who can tell you exactly what plug to use (and what grade of oil is most suitable)... I just try to remember what I've read in other threads.