There were 4 allocations of free shares which were on the following dates:
15th October 2013
9th April 2014
5th October 2015
6th October 2016
You had to be an employee on those dates and if I remember correctly for about 3-4 months beforehand aswell, to be given any shares.
There's always a vague possibility there may be another free shares allocation, especially for those shares surrendered by 'bad leavers', but there's no actual plans to do so, as far as I'm aware.
There's nothing stopping you from buying some yourself via a stock broker. But giving up the employer contributions, tax relief and PSE you get from paying into the RM pension arrangements in favour of investing directly in RM shares, probably wouldn't be a very good move.