Hi Louise89,
They are allowed to ask for whatever they please. However, it does not mean you are obliged to pay it.
From what you have written, paying their legal fees now seems unreasonable, because:
1. The primary debt as been paid, you managed their expectations throughout the debt owing and they informed you that they would not take any legal action, so this is new to you and they did not mitigate their costs (so if the claim is for legal fees, this would be unreasonable);
2. Since the debt has been paid, they cannot pursue this claim for the primary debt so they would not "win" this action; and
3. Usually, a company pays for their own legal fees and expenses.
However, do you still owe them any primary debt? If not, what is the filing and service fee they claim?
By "Statement of Claim", have they actually initiated legal action against you? By this, I mean, have they started legal proceedings in court (and not just spoken with a lawyer about it)? You need to clarify whether it is a "Statement of Claim" (court action) or "Letter of Claim" (no court action).
What type of company is this?