Required to cover colleague's leave

[SeaWitchly]

I work 3 days/week in the healthcare industry (allied health and private sector). I am in a team of two and I am the senior member of staff. I have no responsibility for firing or hiring, I am senior in terms of mentoring junior staff and writing policies, etc.
I have recently been informed by management that I am also responsible for covering my colleague’s annual leave, so working 5 days a week when she is away to cover service provision. The problem is I have other work commitments on the 2 days I am not permanently employed which cannot easily be changed. And to be honest I think why should I? But is this just the reality of being ‘senior’, that I am essentially required to be on unofficial ‘on call’ to serve the needs of my organisation?
What do others think?

Just because I did it before as a favour doesn't mean I have to do it again.

I worked extra days for some years but that was always on a case by case basis and my colleagues fully accepted that I could only do it under certain circumstances.

I would contact them by email, explaining that there is no provision in your contract that you have to do this and any favours in the past were exactly that, a favour on a case by case basis.

Any changes to the contract have to be agreed with you.

Just because you did again doesn’t mean you have to do it again.
It’s not in your contract and you have another job on those two days.

I’m sure that your boss wouodnt appreciate you not turning up for work because you have to cover someone with your other work commitment. There is no reason why he shouod expect you to do that.

Having said that, I suspect he thinks you are ‘just’ part time and therefore have no other SERIOUS commitment outside the work you do for them so it’s fair enough to ask (or is it demand?) you work those other days.
You need to stand you your ground and be clear, in yourself, that your commitments in those two other days are just as important than your commitment to that job.
(I do think that saying you are working those other two days will help. It will be taken as a serious occupation and something you can’t change. I dint agree with that POV BUT if it helps putting your point across, go for it)

if you have a part time contract for 21 hours there is no obligation to work beyond 21 hours
if you are on a job share your contract might be slightly different to you being on a permanent part time contract
no clause in a contract can stop you earning a living legally so if you have a 3 day job there and a 2 day job elsewhere they can't require you to alter second job days for them
opened ended clauses have to be reasonably some one can't be expected to be on stand by for 2-3 days a year
lots of people have more than 1 job and you have obligations to both if you work Monday, tuesday and thursday for company A and Wednesday morning and all day friday for company B , company A can't demand you work for them next Friday nor can they demand you take annual leave from company B to suit them
It is quite common in what was my field a dentist worked in practice A certain days and practice B different days if practice A decides to have a training day when you work at practice B you miss it