So I have made some progress, I realised I could export a modelbuilder version as a script and compare.
The problem was my extent I was passing in wasnt in correct format.
I now have the extent converted to a string and it runs, but it doesnt clip the right area
import arcpy
arcpy.env.workspace = r"C:\Jim\BathySelect\BSelect.gdb"
arcpy.env.overwriteOutput = True
inTif = r"C:\Jim\BathySelect\Data\Bathy\Whole UK Mosaic.tif"
poly = r"C:\Jim\BathySelect\SelectPoly.shp"
desc = arcpy.Describe(poly)
outTif = r"C:\Jim\output.tif"
extent = desc.extent
arcpy.Clip_management(inTif, str(extent), outTif, poly, "0", "NONE", "NO_MAINTAIN_EXTENT")
when I look at the generated script the values it passes in for the same polygon are :-
"-610219.285476707 6629015.89017292 -581115.060601592 6655474.27642303"
but the figures in my extent are :-
“-5.48169310808 51.2315325764 -5.2202454077 51.3805117862”
this looks like a lat/lng vs metres problem now but not sure, still after a bit of help.