Yeah its a cam phase sensor, works on the Hall effect type. There is a ring on the back of the exh pulley with cut outs, so the ECU knows where the cam is in relation to the crank.
It runs fine without it - I broke mine
when i changed the timing belt, I bent ever so slightly the ring and when it turned it smashed to bits
It's about £100 from a dealer (it comes on the mounting plate so is already aligned) - only downside, to fit you have to remove the exh pulley, so belts, etc off AGAIN.
I did it myself, found the hardest bit was trying to get the cabling through, it goes behind a plate behind the inlet cam, and around the back of the head, the steering reservoir had to be removed.
I'm pretty sure when I checked the sensor, it was like a £10 part, sure it was made by Honeywell, i can check if you want