Ariel used to be a pretty terrible dive but it's not too bad since they renovated it. I don't know what the standard rooms are like nowadays but I suspect they'd be quite small (recent stay was in an exec room after booking a single for £35+VAT or something silly like that). But if it's just a bed for the night, it's perfectly fine.

I think it's on Bus 423 from T5, by the way.

Note though that you can sometimes get Sheraton Heathrow for £49+VAT on weekends.