I eventually managed to get my hands on some rhubarb after more than two weeks of searching the shops and markets of Dublin. Finally I spotted a small lonely bunch in a shop this evening - clearly everyone had been looking for, and found, rhubarb before me. I will not complain - off I cycled with my bunch which amounted to approximately 220 grams of useable fruit, just enough for a warming dessert for two.
The recipe listed above makes enough for two ramekins. For a larger crumble, you can make this in an individual pie dish with as much fruit as you like and the crumble ingredients doubled.