A Much-Loved Rachel Trevor Morgan Hat and Purple Double-Breasted Stweart Parvin Coat

This is an often-repeated coat and hat combo, in aubergine wool with black velvet accents. The outfit is by Stewart Parvin, while the hat is a Rachel Trevor Morgan design, which appears to have been slightly altered over the years.
This look first appeared back on 11 November 2009, for a Remembrance Day service at Westminster Abbey, accessorised with the oversized Queen Mary Dorset Bow brooch and the traditional sprig of poppies:

On 3 March 2010 the outfit was recycled for the ceremonial welcome of President Jacob Zuma of South Africa at the start of his 3-day State visit to the UK. On this occasion we first saw the dress that goes with this look, which features the same gold button symmetrical detailing of the coat:

A back half-belt can also be seen here:

This ensamble, minus the hat and with the addition of the Frosted Sunflower brooch, reappeared later that year during the annual Christmas Broadcast, on 25 December 2010:

On 22 March 2011, we got a clearer view of the matching dress usually hidden beneath this coat, when the Queen received General David Petraeus for an audience at Buckingham Palace:

On 25 October 2012, at the opening of the Jubilee Gardens on the South Bank in London, the hat brim appeared noticeably smaller and the placement of the coat buttons appeared different as well, with the top two next to the lapel having been removed. It was a rainy day, so a purple-edged umbrella was called for. No brooch:

A month later, on 26 November 2012, the look was repeated on a visit to the Household Cavalry at Combermere Barracks, near Windsor. Predictably, the Household Cavalry badges were worn:

The Queen was once again pretty in purple when attending church in West Newton, Norfolk, on 3 February 2013. Again, no brooch - this seems to be an outfit that doesn't lend itself much to the showing off of jewellery!

A few months later, on 13 May 2013, it was worn again at the Royal Windsor Horse Show, this time aptly accessorised with the Kent amethyst brooch:

Later that year, on 21 November 2013, the Queen wore this outfit again on a visit to Southwark Cathedral in London, this time accessorising with her amethyst bouquet brooch:

Later the same day, the Queen and the Duke of Edinburgh visited the Shard, the tallest building in Western Europe:

It was seen again in public more than a year later, when the Queen wore it for a pre-Christmas church service at St Mary Magdalene on the Sandringham Estate on 21 December 2014. The Kent amethyst brooch was used again, althought it can't be seen clearly in the photo below:

On 27 February 2015, we got another look at the dress, when the Queen and the Duke of Edinburgh received the King and Queen of Tonga at Windsor Castle. The FEI (Fédération Équestre Internationale - International Federation for Equestrian Sports) Lifetime Achievement brooch can be just seen amid the gold buttons:

On 21 October 2015, the Queen wore the coat and dress for a quick afternoon appearance at the Royal Mews in Buckingham Palace, where the famous horse whisperer Monty Roberts was giving a demonstration (Daily Mail article here). No brooch was used:

On 15 January 2017, the Queen repeated the look to attend church at St Mary the Virgin in Flitcham, on the edge of the Sandringham estate in Norfolk (Daily Mail article here). Again, no brooch:

On 10 February 2017, the Queen wore the dress on its own to hold audiences at Buckingham Palace (below, she is seen with His Excellency Mr Abdesselam Aboudrar, the Ambassador of Morocco, accompanied by Mrs Saloua Larhrissi: