Can I just clarify the position regarding being a Trustee vs being the Chairperson in your case
For the Pre-school I chaired, we had a group of Building Trustees [technically custodian trustees] (we owned the building, and this group held the trusteeship of that building) and a committee, who were the Charity Trustees of the Pre-school itself.
In your case, when you say you are a Trustee, is this that you are a Charity trustee? And is the Chairperson the Chair of that Charity trustee group? I just don't want to confuse things...
If the Trustees are Charity Trustees, then you have fairly serious legal responsibilities (for example you are jointly and severally liable for financial losses incurred by the charity if they are incurred because of your mismanagement or because you acted in bad faith, you are the employers of the staff etc). It is therefore normal to be involved in discussions about pay rises etc as you employ the staff and have the responsibility to ensure that such rises are affordable..
It is the Trustees' responsibility (severally) to ensure that the charity is properly managed, and that staff employment is in line with employment law. You could delegate staff appraisals to the Manager (because you can delegate some aspects of running the charity day-to-day to employees), though you should do the Manager's. You should be involved in recruitment. As a committee, you must have a good handle on the finances through a clear budget managed by the treasurer.
As Chair, you do not in fact have greater legal responsibilities than you doi as a Trustee. The main difference is that you tend to be the 'front man' for dealing with staff.
If I were you, I would read
a) the constitution of your pre-school. This may well be in the Prospectus, otherwise ask for it.
b) Guidance from the charity commission www.charitycommission.gov.uk/Publications/cc3.aspx#d4
I would then consider whether the issue with the current Chair is because the manager and staff are used to 'managing themselves' (in which case the problem is ongoing, as your legal position as Trustees makes this a state of affairs that cannot continue) or because the current chair is particularly abrasive. I would also look VERY carefully at the accounts - managing a solvent pre-school with a healthy revenue stream, a number of good fund-rasing activities and a full roll is very different from managing one with a falling roll, not quite enough money and lurching from one financial crisis to another! During my period as Chair, we faced a big problem because the neighbouring school wanted to shift to a singe entry in September, whereas they had run 3 entries, one each term. In the end, a compromise of 2 entries was reached and that kept our revenue stream healthy. But that kind of thing can put a big hole in a previously balanced budget.
Funnily enough, I went on to retrain as a teacher....