Anyone know how to attach a filter ring to this lens?

I use a Lee system, including a 49mm ring, so if there is a push fitadapter to 49 mm or so that would be great. Anyone know the diameter of the outside barrel?

I did see an answer to this question somewhere on the web ages ago but have now given up after a 2 hour search trying to find it again!

I am UK based so if anyone knows of where I could get an off the shelf solution, I would love to know.

Thanks

Tom