What's causing this on my Rhododendron?

One of my Rhododendrons has browning around the edges of some of the leaves. Can anyone tell me what's caused it and how I can treat it?

 

I've found this link which gives some info - basically it could be under watering, over watering, too much sun, etc. It does get late afternoon sun so will move it under a shadier part and keep an eye on the watering.

Any other ideas?

 

could be the overwatering or sun as mentioned in the link

let it dry out, then water well once a week or every 10 days .... also spray the leaves (top and bottom with a liquid seweed fertilizer once a month) .... use something like maxicrop seaweed extract with added iron:
http://www.rhododendrons.co.uk/Product/17123/maxicrop-seaweed-plus-iron-1lt.aspx

another good feed is liquid fish hyrolosate .... but you may need to buy that off ebay, as not many garden centres carry stock

this link also makes mention of browning caused by fertilizer:
http://www.rhododendrons.co.uk/Category/68/plant-food.aspx
snip:
Rhododendrons are light feeders, so we have selected slow acting fertilizers which will not scorch the leaves when used as directed. We recommend Miracle-Gro Slow Release Fertilizer and this is similar to the main feed we use on the nursery and lasts the whole season. Vitax Conifer and Shrub is also a good product which lasts 3 months, while the liquid feeds are excellent for quickly greening up any poor looking plants. Other products shown are useful to correct deficiencies in your soil.

 

Hi Dim.

I've already mulched and applied some Miracle Gro slow release fertilizer for Rhodos and Camellias, plus a couple of weeks before I did that, it had a foliar feed with Vitax seaweed with sequestered iron. Funnily enough, I got the Rhodo from Millais Nurseries which you've linked to

It's in a container and we haven't had a decent downpour for weeks, so I give it a good soak every few days.

I was looking at it earlier and although it is on the top patio by the pergola, it's on the south facing side but it is shaded somewhat and currently it's in dappled shade, which is ideal.

It's only been like that since winter, so I wouldn't have thought that we'd had enough sun yet to cause the damage, so maybe I didn't water it enough in the winter. I sometimes forget that even if it rains, it may not be enough to penetrate the canopy and get to the roots of the plant, especially in a container.