Presumptions - things we already think we know.
1) A presumption against the change in the common law (judge made law, from cases), unless an Act overrules it.
Leech v R - Women could not be made to give evidence against her husband. This was changed by PACE Act 1984. (PACE = Police and Criminal Evidence).
Judges have to follow rules made by another judge in a case, unless Parliament have overruled it.
2) A presumption that 'mens rea' (mental element - intention) is required in criminal cases.
Some minor offences are called 'strict liability'. The dft can be guilty even though he didn't intend to do the Act, but for serious cases the dft must have some level of intention even if the Act doesn't specify it.
Sweet v Parsely - Dft rented house out to students who smoked cannabis there without her knowing. The Act did not mention any intention, so the judge presumed some intention was required, so this made the dft not guilty.
3) A presumption that the crown is not bound. - monarch doesn't have to obey law unless the Act says.
4) Legislation does not apply retrospectively (looking back).