During my trawls for information, I came across: "But what about using something other than DD-WRT? Our friends over at How-To Geek have a
guide on creating a repeater with Tomato, but it’s more of a workaround as Tomato doesn’t support this functionality explicitly. ". Taken from:
http://lifehacker.com/how-to-extend-your-wi-fi-network-with-an-old-router-915783308/963787201
And delving into the How-to-Geek link they give a How-to-Geek:
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"Last year we showed you how to
extend your network using DD-WRT powered routers. Since then, several readers have written in, asking how they could do the same thing with Tomato routers. Many of them said they’d switch to DD-WRT if they had to, but would prefer to keep using Tomato if possible. Although Tomato doesn’t have a perfectly analogous match to the DD-WRT repeater mode (more on this later) you can easily link two Tomato routers together with only a few minutes of configuration."
However, linking 2 Tomato routers is not what you want. Now, it's possible the features you want have been incorporated into Tomato since those articles were written - worth a trip to their forums to search/ask.
On the other hand, don't give up on DDWRT: it may well be I was doing something silly (won't be the first or last time) or there was some problem with my wireless channels. If I'd persevered it may well have worked. (In case you do look at DDWRT, the firmware on my WRT54GL is V24-sp2(08/12/10)vpn, which I believe is the right one for it; finding the correct build takes a lot longer than installing it and getting it running, and that's not exactly a piece of cake.)
Let us know how you get on if you do solve it. Good luck.