"Bad ambulance driving"

Thursday 26 September, about 7: 05pm, junction of Thirlestaine Road and Old Bath Road, Cheltenham.

I was turning right at a small roundabout, giving way to traffic from the right. The view of traffic approaching from the left is obscured by a wall and trees (but these vehicles should give way to me, so not a major problem).

I pulled away to make my turn, only for an ambulance to suddenly appear to my left (emerging suddenly from the point where my vision is obscured by the wall), without its siren on, but apparently expecting me to give way to him/her against the priority. He/she had blue lights on, but no siren. Because of the wall and trees, the blue lights were not visible from my position until right at the last moment (when I'm looking to the right, pulling out, and in the middle of the roundabout). The ambulance appears suddenly to my left and blasts his horn at me for not giving way!

This is a bit stupid and dangerous (and annoying). If an ambulance wants to turn right against the give-way priority at a badly sighted roundabout, it needs to have the siren on to warn of its approach. Please could you reiterate this to your drivers.