Time off (sickness)

Affynity

So, I've had problems with my wisdom teeth and thanks to Easter, I couldn't get in to see my dentist till yesterday. I won't be able to get any treatment until Friday because my emergency appointment was late in the day. I got antibiotics though, so hopefully they'll help, although I doubt it because it's impaction pain.

I'm contracted to work Monday and Tuesday this week, so there was no working Bank Hol Mon because we weren't open and Tues I took off sick.
I had been rota'd for overtime on Weds & Thurs, which I've also taken off.

So I'm wondering now, do I need to self-cert for one day?
My contract is a little ambiguous when it comes to holidays and overtime.

On overtime:

"Your contracted working hours are 16.00 per week, together with such additional hours as may be required to meet the needs of the business. There will be no payment for additional hours unless agreed in advance with your manager."

On holidays:

"Your entitlement comprises company and bank holidays. You will need to save enough holiday to take cover for:

bank holidays

other exceptional closure days (e.g Christmas day, Easter Sunday)
when you would normally be working and your place of work is closed.

It is a condition of your employment that you work on a bank holiday when required."

So, from what I can gather, the overtime is mandatory but the payment isn't, which I can only assume results in time off in-lieu.
Being closed for bank holiday appears to be my problem and comes out of my holiday entitlement?

My manager is asking me to self-cert, but I'm not entirely sure this is appropriate because it's one day and not 7.
I'm being made to feel like this is a big issue, but I'm not quite seeing it. Any advice on this? Is it common (or necessary) practise to self-certify on additional, unpaid hours?