In reply to SambucaHughes:
There are a few different issues here that people are (understandably) confused over.
For most minor offences there is a period after which the conviction is 'spent'. This means that it is illegal for an employer to ask about it, and it will not show up in some of the more basic kinds of record check. Roles with children, vulnerable adults and in other kinds of sensitive activity are exempted from the relevant legislation - they have to declare this on the advert. For these kinds of roles, they will perform an enhanced disclosure. An enhanced disclosure will show up everything that has ever gone onto your criminal record (including cautions) if the Police think it should be released (see below).
Separate to this is the question of 'stepping down' and/or of discretionary withholding of information by the Police. Stepping down is a process by which old conviction information is simply removed from the database. This happens to eg. very old convictions for shoplifting or possession. It's done by individual police forces as an administrative 'tidying up' measure. If this happens, there is no active record at all of the conviction. It will never show up anywhere (even if you went to court and were sentenced for a new crime), because the record has been buried. More recently, Police have been encouraged to filter the information that is disclosed, even for enhanced criminal records checks. They have to make a call on the relevance of a conviction. This is a bit more random, since forces can take different approaches from one another and even from case to case. They may decide that your cannabis conviction is not relevant for some positions, and in those cases it will simply not show up on your disclosure. The 'continuous update' system messes this up though - new convictions etc will still be obvious to the employer.
Finally, and to the OP - you will gather from the above that, in most cases, your offence is likely to appear for a good while yet when you apply for most posts that involve working with children. You have to hope that the folks doing recruitment have the knowledge and sense to assess the risk in a pragmatic way, and to go beyond a knee jerk reaction. It doesn't always happen. If you are under consideration for a post, remember that they will inevitably be doing some guesswork about the type of person you are. You may need to do more than other candidates to defeat the cannabis/druggy stereotype. It's a real pain, but remember that lots of people have cartoonish views, so appearing smart and organised, and talking about the need to be a clean-cut role model for your young clients will go some way to defeating people's prejudices.
Finally, I used to work doing various forms of vetting and licensing across the sport sector, which is where my knowledge comes from. I saw lots of panels in discussion on these issues, and saw how different factors affected the outcome. Convictions are not uncommon, and I would bet that outdoor centres must see lots of cannabis offences on disclosures. Unfortunately, it is very easy for them to take a completely risk averse view and just turn down the application. On the other hand, they are unlikely to be genuinely shocked or alarmed. As others have advised, any kind of work history can have a huge effect on cutting through the problem - showing a good record of hard work with a decent reference has huge value, as it allows the panel to convince themselves that they are not taking a gamble.
Good luck.
Mark