Posting so others don't make the same stupid mistake. 
I spent all of yesterday lying in the shade of my large patio umbrella. I vaguely knew that a tiny amount of sun still reflects on to you in the shade but thought this might lead to a light tan at most. But by evening I'd gone red and quite sore. I'm especially cross with myself as I've burned my chest (in a delightful v-neck formation
) and I'm usually so careful to cover my chest due to the skin being thinner and more prone to wrinkles later.
I've had a quick look and it seems proper UV-blocking parasols are quite expensive so I suppose I will be wearing sunscreen when sitting under it in future, unless anyone can suggest anything else? Maybe I could throw some kind of thick tarpaulin over my existing parasol?
Anyway thanks for listening to my public service announcement.