Change in working conditions...

I have worked for the same company for 17 years. Because I have a disabled child it was agreed that I could work from home and that I would be paid 50% of the full working week.
The company has recently made 30% of its workforce redundant. There are 3 admin staff - 2 secretarial and myself. I do the bookkeeping including all staff salaries.
I was called into a meeting today and asked if I would consider not working from home in the future but coming into the office as they want to make one of the fulltime secretaries part time. Their inference was that I should do both the work that I do now and cover the hours that the now full time secretary would not be there - i.e. my job and general reception duties as well (as yet actual hours have not been discussed). They also want me to take a reduction in salary (although they called it a reduction in hours) effectively doing the same work but in the office and extra duties such as answering the telephone, etc. My responsibilities to my disabled son are such that I have to be home for him being dropped off by school transport and I have to be available for any appointments and emergency calls.
What would be my position if I said I would prefer to continue as I am - working from home - on the basis that it would be virtually impossible for me dedicate myself to office hours because of my situation? Can they force me to change my working agreement? Although I have been working in this position for 17 years I'm not sure if I have an actual contract outlining this agreement.

Sorry I have been so long winded !!! Any help/advise would be much appreicated.

It would be a companies decision to force you which I think they would, but it would be your decision to accept it or not. Just inform them about your problems. I don't think that they could pressurize you for this or threaten you to leave the job. Have a talk with them and see how the situation turns out to be.

It doesn't matter if you have a written contract stating your current working arrangements, if they have been consistent for a long time they become part of your terms and conditions so cannot easily be changed without your consent.

So you can refuse to change, however you should explore with your employer what the alternative is. If they are proposing this in order to avoid making you redundant there's nothing wrong with doing that. You need more information so you can make an informed decision.