Trying to keep things fairly vague whilst still getting the key points across.
In my job I have additional responsibilities which I am paid for in each pay cheque however all the work is in the summer months. It is about £1000 in total. Since money is going to become tight whilst on maternity I need this additional money.
I am due at a time which means I am unlikely to be able to fulfil this role. I will still be able to do parts of it though. Can they stop paying me for it or are they expected to find alternative work which I can do?
Are the additional responsibilities part of your job description/contract?
I was under the impression that you are able to request reasonable adjustments during pregnancy, but I'm not sure from your OP if you mean you will already be on maternity leave when this work would ordinarily be carried out.
Not sure what job you do, but are you a union member? Or if you're not could you ask ACAS/CAB for advice?
I'd think that you can argue the case for reasonable adjustments, especially if it is a role which you ordinarily do each year. I've had a lot of support from my union to get bits sorted as they were able to give me a definitive answer on what I was/wasn't able to demand request so at least I knew I was right when I was arguing the point IYSWIM