It is normal invoices (all kindsm, not just solicitors) to have a time limit for bills to be paid.
chasing those bills takes time and money - it''s not unreasonable for the solicitor to expect you to take responsibility for paying your bill.
From what you are satyying, it sonds as though you had one bill for £100 in March, which you haven''t paid, despite knowing that the terms were for payment in 28 days, and have now received a further bill for £300.
With regard to the first one, I think it is absolutely reaonable for the solicitor to charge interest on it - after all, they have effectively lent you £100 from 28 April (when the bill was due) until now. They are running a business, and that busienss is
notprovidng loans or managing your finances.
With regard to the 2nd bill, I''m not quite clear what this is for. Is this a bill for money which your ex was going to pay but hasn''t? If so, then your quarrell is with your ex, not with the solicitor / mediator. When you instruct them, you accept responsibility for paying the bill. If you agree with your ex that she will pay part of your bill, that is an agreement between you and her, it doesn''t affect the relationship between you and the solicitor/mediator.
Interest doesn''t run on a bill until it is riaes, so interest on the £300 would only run if you don''t pay it within the 28 days. If you can''t pay it then you need to talk to the solicitor to see whether they are willing to agree to accept payment by installments.
if you are unhappy with the way in which the mediator dealt with the
mediation, then you need to raise that with the mediator in the first instance. However, do remember that just because mediation wasn''t sucessful, doesn''t mean that the mediator did not do their job properly - so you would need to be very clear about what you felt was wrong, and how/why you feel this is due to faults on the part of the solicitor / mediator.