In reply to jon:
grated gullies are (potentially) as much of a problem as a solution. they used to be the common method of taking gutter downpipes below ground; but fell out of favor sometime in the 70s. the problem can be, you get as much crap, leaf mulch, windblown debris etc. into the grating as washed down the downpipe. a hopper at ground level is.... a potential maintenance problem. (grates still let a lot of stuff through b.t.w)
more important is access and rodability. if you connect direct to the slotted drain then install a rodding eye too, and check where it goes at the downstream end so you know where it'll rod out.
does the gravel "grip" extend around the perimeter of the building? If so, I suspect the "soakaway" may not be a soakaway but a gravel perimeter drain, to take water away from the buried brickwork of the building. the reason is, if it were a soakaway I would expect the slotts to be on the bottom of the pipe, or all around, to disperse water from the pipe to the gravel. (soakaways have a minimum distance from buildings requirement; 10m?, can't remember of the top of my head) if it's only slots on the top of the pipe, then this'll flow pretty near pipe full before water will escape out.
i'd be inclined, if you're going to connect directly, to ignore the slotts. only in proper pipe full flow will water escape, and during these events everything will be wet in any case. The risk, otherwise, would be actually preventing the gravel from draining
Post edited at 19:18