I've a small piece of topside/silverside beef (under 1kg) for tea tonight, however am going out and need to leave it to cook low and slow for most of this afternoon.

Is gas mark 1 going to cook it across four hours..? Or should I put it in the middle of the oven at mark 2, covered with foil and with a drop of water in the bottom of the pan..?

Anything other than onion that I can put in the pan to help with the gravy/flavour?

I'd have popped it in for an hour before going out. I think a long slow cook will kill this joint as it doesnt have a lot of fat content. Could be wrong though

I cook mine in a large casserole dish. I crumble a couple of oxo cubes into a jug, add a little hot water to them to make it into a paste, and then, add about 3/4 to a pint of water. Pour it over the beef, season with salt, pepper and Worcestershire Sauce. You can even add a half teaspoon of mustard to the pot to give it a richer taste if you like.
Then cook it really, really slowly, say about 3 to 4 hours. I've never had a bad roast beef joint that way. Always beautifully tender, and I use the remaining liquid to make the gravy!! Scrumdiddlyumptious!