The car sold in UK does not have a small engine with turbo - that is JDM only. Those in UK have normally aspirated engines of (I think) 1300 cc.
I'm afraid the mechanic is probably correct in diagnosing the ECM. The usual diagnostic test as given in Suzuki FSMs is to "replace with known good ECM", and see if the symptom goes away.
It is possible to get 2nd hand parts on Yahoo auctions in Japan. Of course, it helps to read/write Japanese, but you can use an online translator.
Alternatively, go to http://www.j-garage.com
, and contact Yoshitaka Ikeda. He speaks good English, and a Russian friend at the Uni of Beijing (!) recommends him highly.