Tumble Tots is great. No-one speaks to me when I am there but it doesn't really matter because it is such a short session and you're not really there to socialise like with a playgroup.
I'm sure there must be a music class in your area, they have sprung up everywhere.
Are you a member of the NCT? Our local NCT booklet, that we get every 3 months, lists activities in there.
Have you got a big health centre near you? Their notice boards will often list preschool activities, as will the library noticeboard.
Do you take your son to the library? My local library does a bump and rhyme session every fortnight for children not old enough for story-time. Also, the library is a good place to go when it is raining. Me and my dd can kill a good hour of time in the library. And on top of that, a large library will advertise toddler activities.
Ask your HV too. I ma sure there is a lot more going on than you realise. I agree, it is difficult when there is no one central place for mothers to look for this information.