SIPP - Self Invested Personal Pension Plan
This term is now redundant in its technical pre-April 2006 sense.
What you need to know post April 2006 if you have an existing contract of one of these types.
It will continue as an Arrangement (type subject to scheme details), and you/your employer can invest as much as you like, subject to the Annual and Lifetime Allowance rules.
Investments, Borrowing, Anti-Avoidance - the new rules are a little different, discuss with your Financial Adviser, especially if your scheme has any existing property or moveable assets, or wishes to enter into a connected persons transaction.
If you have a policy of this type, begun before April 2006, bring it to us and we will be able to find out its current status and classification.