Thanks for the response EvilDoctorK.
I am travelling on a ticket booked with BA, but the Aer Lingus portions of the flight is an EI flight number, EI382.
I have a booking reference for the BA booking, which I can use to view the flights on the BA website (all that allows me to do is request special meals, no option to select seats or check in online), but I also have a seperate booking reference for the Aer Lingus flight (it's the same format as I've gotten before when booking online with Aer Lingues, 6 characters, mix of letters and numbers. But as I said, I can't use this to access the Aer Lingus website.
Thanks for the advice on the BA luggage policy - I was aware of it when I was booking, but I don't plan on travelling out with a huge amount of luggage (infact, the restriction is probably a good thing, since it'll stop me packing loads of useless crap). From looking at the BA website, after September 2007 the allowance will be 20kg, but for flights until that time (including mine, I think) it's still 32kg "to help customers adjust to the new restrictions". The only time I can see this being a problem is on the return journey. I definetly don't want to be asked to pay a few hundred euro extra for a few kilos.