Edit: Could be "Children of the Dog Star"(NZ 1984) which aired in Britain in August '85. The show features children in a rural setting, it's SciFi and includes something in a swamp with circular shape, which is recovered by the children. Below is the recovery from the swamp and assorted mystery devices from the show:
Source: http://www.frequency.com/video/children-of-dog-star-1984/101862698/-/5-6263323
ItEdit: First guess was that it could have been a re-run of "Here Come the Double Deckers" from 1970-71. It takes place in a junk yard in London where a group of children live in an abandoned double-decker bus. It features a fair bit of tinkering and a robot in at least one episode. It was a joint UK-US production, which might explain the accents you remember. It was a deliberate attempt at creating something along the lines of "Our Gang"/"The Little Rascals".
What the writers tried to emulate: