1. Piece of string, but probably long enough if not too long - how long would your garden lawn get if unmowed for 6 weeks?
2. Subcutaneous, yes. Often leaves a lump, so do it high up on the neck behind the ear, if you can - avoids getting nicked on shearing (next year for any retained lambs but later this for older sheep) and avoids any deductions / condemnations of better meat at the abbatoir.
3. Livestock marts normally keep records of weekly store lamb prices - look at your local mart's internet site, if not there, give them a ring and ask, or check the marts pages in your local paper (from appropriate weeks last year.) They vary across the country, but for example
here's my local mart's store and breeding sheep report from 7th August 2012