HairyDB, good hearing from you 'cross the pond...! Likely simplist problem is fouled plug, but at the same time one of the toughest ignition problems was high resistance in one of my "aged" plug wires on one of my older bikes, back in the day.
Since 5284dva has a 2008 XL modern EFI & more robust ignition, I would go with simplist problem/solution plug replacement/swap first. Another would be check that the ignition wires connections are clean and seated fully in the coil and boots to the spark plug, sometimes that terminal atop the plug unscrews so make sure they are tight and torqued properly using new standard HD plug (no substitues).