Yes you should call them. It's very simple, enter their phone number into your phone and press ring...
If you want your limits raised you can only do this once every 30 days, if your last limits increase was in this period you will be refused but try anyway. They are quite good.
They will ask you:
1. What you sell
2. How much stock you carry (just lie)
3. How many people work for you (again, lie)
4. They may need a VAT number as it looks like you will pass the VAT thresh hold
5. They may ask for a Ltd company number
Feedback must be above 98% from memory, and your defect rate must be under 5%
Otherwise, you should be good to go.