Squealing chirping noise from drive belt c4 gp 12 plate

Hi everyone looking for some thoughts please !! C4 gp airdream 1.6hdi auto. Due for service and m o t . 2012 plate 35k. Today doing school run and started to hear a squeaking chirping noise from under bonet. Lifting bonnet the noise coming from the drive belt and pulley area. On acceleration it kind of goes off. Also the more electrics / air con I put on the louder it is. Service light came on and warning fault repair needed came up and a warning saying eco switched off. On increase in speed they all went off . Sat car in drive on idle and it was making the noise faintly. Turned air con on and lights and it started louder. What are anyone’s thoughts and is this going to cost me £££. Read it could be there belt then maybe tensioner !??? Any ideas greatly appreciated and what sort of
Job to fix / price if any ideas !!!
Thanks in advance