The crux of their argument is that the Iraqi resistance's relatively new use of shaped charges and of laser -- and not radio -- triggered detonators constitutes complicity from the Iranian government. These techniques, which mirror techniques used in the past by Hezbollah, do not, in themselves, indicate active Iranian involvement in Iraq.
That said, the Iranian government does legitimately have financial and military influence on Iraq. Both the Badr Brigades and Moqtada Sadr's Mahdi Army received training, funding, and were strongly ideologically influenced from Iran, and it is, indeed, quite possible (and probably likely) that Iran is doing their best to influence the future of Iraq. That said, directly connecting Iranian government involvement in the training of Iraqi insurgents would be a stretch, especially given the glut of information available through other sources, such as online, from former Soviet-bloc countries, or via the numerous Hezbollah training videos that are floating around out there.
Making an improvised explosive device with a laser trigger would be a laughably easy thing to do, and parts could be acquired in bulk, not only from companies that sell electronics and security equipment, but also from commercially-available toys for children. Shaped charges are technology dating back to World War One, and are used extensively in the oil and gas industry to open up the rock around drilled wells. Neither shaped charges or laser triggers are advanced technology, and there is a wealth of information available out there on how to construct them. Shaped charges do, however, require several weeks to fabricate, and cost about four or five times more than bulk explosives if you buy them from the likely sources -- in this case, from those involved in the oil industry who might sell them via black market deals.
In this case, it's entirely possible that Iran, Syria, Jordan, Kuwait, Saudi Arabia, Turkey... all of these countries might be home to people who want to contribute arms to or profit from the war. The entire Iraqi border is pourous, and, in those areas with Iraqi guards, a well-placed bribe can get truckloads of explosives into the country. For the British Prime Minister to single Iran out, without providing intelligence to indicate direct complicity, seems more than a bit presumptive. More than likely, such claims have more to do with Iranian insistance on their right to pursue a nuclear energy program, and far less to do with lasers and shaped charges.
In other words, the British have tried warnings. Now they're resorting to threats. What's worse is that they are giving voice in their papers to people like Michael Rubin, a neo-con of questionable loyalty, who helped run the infamous Pentagon Office of Special Plans, which was used to stovepipe flawed intelligence to support the war against Iraq.
The Guardian should know better. So should the British people.