Painted by Dobson, Sir Simon was a royalist who was captured during the war and spent the rest of the 1640s in exile in France. He was also an associate member of the Sealed Knot during the interregnum.
Sir Simon wears a sleeveless buffcoat with pointed waistline, spirally laced and rather similar in style to the one held in Kensington Palace, reputed to belong to the King. Here’s a link to a blog post about it. The sleeves are embroidered or maybe laced (meaning braid), and may be part of a doublet or may just be attached to the lining of his leather coat. He has a falling band which is half linen/half lace and matching, though not identically coloured breeches. The colour of the breeches suggest that the sleeves are not part of a doublet/breeches combination but attached to his coat. He also has an almost impracticably long sword baldrick, which is fringed with metallic threads that match the decoration on his sleeves.
The portrait hangs in the Valance House Museum in Dagenham Essex.