I've heard conflicting sources, some, (such as Wikipedia) saying 1986, while others say 1987. I haven't been able to find it in any TV Times listings pre-1989, which I know by then it became a permanent fixture in the lunchtime slot.
The Wiki article on Sesame Street in the UK gives the correct date of 30 November 1987.
The Liverpool Echo (7 Nov 1987) stated that Sesame Street would be shown on Channel Four ‘every lunchtime’ from the end of that month.
The Stage (26 Nov 1987) reported that the series would commence the following Monday (30 Nov) and was the 18th season which ’was shown in the US last year’.