I know some crafters worry about this, so i thought I'd post this.
Dh this morning got through a letter from IR saying that because his income from SELF EMPLOYMENT (not including his proper job) is below £4345.00, he has an exemption for small earnings certificate and has to pay nothing.
Therefore, you DO have to register (phone or internet- 10 minutes), but you don't have to pay anything on average crafters earnings.
HTH someone.