Steve, my missus works for MoneyClaimOnline - in fact she troubleshoots problems and queries.
Her advice is if you have not yet submitted it, you can go back to your claim and amend the details. Use the back button (?)
You are allowed to charge reasonable interest retrospectively, so no problem there. At the top of the input page there is a tickbox asking if you want to claim interest. When you tick this it inserts a 6 line long paragraph in your claim details. You only have 24 lines total, this may take out some of your particulars if you originally used all 24 lines so make sure you preview the whole claim before submitting it. You will also be asked the amount of interest. Note that this is the amount of money per day, not the percentage. The website says interest but it actually wants the pennies - this is a bug in the website. Your interest on £100 is £0,02 per day on £100 (£100 * 0.08 / 365 = £0.02) so enter 0.02 here. Note that this 2p per day will only accrue from the date of your claim, so you need to change the amount of your claim to include 2p per day interest up until the date of submission of the claim. Further advice is to read the preview of your claim carefully - particularly as the interest tickbox inserts a standard paragraph.
Good luck