I've searched on this Forum and the Aer Lingus forum but still unsure on a baggage question. Grateful for any insight...

I've booked through BA on a Belfast City to Heathrow flight operated by Aer Lingus. It's a 125- ticket number, and my email receipt from BA shows my baggage allowance as a checked bag of up to 23kg but that the hand baggage is subject to Aer Lingus restrictions. There is also a note that Aer Lingus may have different baggage restrictions

Predictably, clicking on the link within the BA receipt to info on Aer Lingus hand baggage just takes me to the Aer Lingus main page and no further. But putting in my booking reference to their site tells me I have no hold baggage allowance and can check hand baggage of up to 10 kg into the hold:

Other info on the Aer Lingus site tells me that they have recently changed their hand baggage policy and that hand baggage up to 10kg can be dropped off for free at check-in with the option of an additional charge of £5.99 to take the hand baggage into the cabin. But this has to be done in advance or it will cost £35 if left until the airport. There's no on-line option within manage my booking with either BA or Aer Lingus to pay the additional £5.99...

My plan is to take hand baggage onto the plane of less than 10kg. If the bag is more than 10kg I will check it into the hold. Questions are:
1/ I assume I have to pay the Aer Lingus cost. Is that correct? I'm BA Gold but think that makes nil difference
2/ How do I pay the £5.99?
3/ If I want to to take a checked bag of, say, 22kg, what is my allowance?

I understand I could call Aer Lingus and/or BA but suspect I might get a clearer answer here!

Thanks for any help.