It's a Whitney Simplex gas engine made to the Bathe design, dates to around 1908 to 1910 and was made in London.
Can you take a picture of the underside of the cylinder so we can see if the valve gear is still there. The value without the valve gear would be somewhere in the region of £400 to £450, with the valve gear about £750, but this depends on the mechanical condition, which is difficult to assess from photographs.
There is a article on Bathe engines in the Stationary Engine Magazine, August & September 2016 issues.
Here is a nice picture of the ad from 1908, £5 in 1908 is around £500 today.
David