In January 2018 HMRC changed the rules on SEIS/EIS Advance Assurance applications so you now need to include the name and address of at least one potential SEIS/EIS investor. If you do your Advance Assurance application via SeedLegals we'll prompt you to enter that information.
HMRC also request that you provide them a 3-year business plan at the same level of detail that you would ordinarily provide to your investors. It doesn't need to be anything fancy, basically just showing an estimate of costs, revenue, P&L over the next 3 years.
If you already have that as a slide in your pitch deck (which you'll send to HMRC as part of your Advance Assurance application), perfect, otherwise you can provide it as a separate document or spreadsheet.
HMRC have made some sample templates available here: https://www.gov.uk/write-business-plan