If your sole modus operandi as a GP is as a locum, and are contributing to the NHS pension scheme, it's pretty much guaranteed that you'll be contributing on the top pension tier.
But the BMA have come to the rescue with some clarification on this. In a nutshell, so long as you're working with an out-of-hours organisation (OOH) for the entire pension/tax year and contributing to the NHS pension scheme as a Type 2 practitioner (usually this involves using Form SOLO), "...this will lead to those GP locums having annualised income equal to actual pensionable income, as they will be considered in the scheme 365 days of the year. This may then lead to a reduction in employee contribution rates for locums."
So even if you do one or two sessions where you have a "...contract for services with the OOHs provider, which is continuously in place for the whole scheme year (1 April to 31 March)", you won't need to annualise.
If you've not yet worked for an OOH provider during this period but were to do your first session tonight, this wouldn't apply to you as you'd have had a 'break in service' since 1st April. But it would mean that next period, assuming you do at least one session for that same OOH organisation next year, you'd benefit in the next period.
Since publication, the BMA have published a blog on this issue.