There are two different R&D schemes you can apply for, depending on the size of your business.
The SME scheme:
The SME Scheme is designed for small companies with less than 500 staff, a turnover under €100m, and a balance sheet under €86m.
The RDEC scheme:
RDEC offers R&D tax credits to larger companies with a staff headcount over 500, a turnover of €100m or more, and at least €86m or more in gross assets.