The small heath is certainly a small species and that can help identify it from other browns. The underside of the wing is quite similar to the meadow brown and the gatekeeper so in this case size matters if it is at rest with its wings closed. The small heath has several overlapping broods each summer so the can be seen almost consistently from May right through until October in mainly grassy areas on downs, cliffs and heath. The small heath is probably the most common grassland species over the course of a summer but it may be outnumbered by Meadow Brown in mid-summer.
Here in Dorset they start to emerge in week 18 which is around the second week in May. There is one report from week 16 in late April for 2017. There are then reports in most weeks through until week 43 at the end of October but there is a break three week break between broods from week 30 to 32 in July. The number of reports is greater in late summer than earlier in the year and I think that reflects the fact that they do seem to be more numerous later in the season.
The are records of small heath from many sites across the county with the dominant vegetation of those sites being calcareous grassland and also heath.