[B'master] We may have agreed that a maximum of 2 'on the go' is acceptable, in which case it's only fair that rab or Darren can post a new toon being as how they are the latest correct guessers.
A clue to No 18 - I've attempted the main [and most recognisable] theme of my 70s TV show - there *is* an introductory bit if anyone needs more ...

No need - it's The Good Life.

[rab] erm ... no. I'm now thinking I haven't done it very well.

OK then... it didn't quite fit!

OK...
plink plingyplink, plink plingyplink
dada daa-daa-daa-daa daa-daa-daa-daaaa
daa-da-daaaaaaa
dada daaa, da-daa, da-daa, da-daa
da daa-da-daaaaaaa
dadada-daa-da-daa da-daa daa-daa
didididi-dee dee-deeeee-deeeee
dididi dee di-dee di-dee da daa-da-daa
da daaaaaa-daaaaa-daaaaaaaaaaaaaaaaa
hahahahahaha!

That was number 19, of course.

[Chalks}Would it be the Sweeney?

[Darren No 19] the Perusaders perchance?

[Dazed5] Sorry, nope.

Perhaps I should say the "hahahaha!" is part of it.

[Dazed] Yup - certainly is.

[Darren] Rentaghost methinks (Gadzooks mistress Meeker)

Classical, in the loose rather than the strict sense of the word.

Plang-chong. Perlang-chong. Plang-chong. Plang.
Plang plong pli pa-dang!
Bubadidididida bubadidididida (continues underneath)
Daaah-di daaah-di daaaah-do ya-di-da
Daah-di ya-di-daaah do daah do-ya-da
Daaah-di daaah-di daaaah-do ya-di-da
Daah-diiiiiii di-yaaah-di dooo

Can supply more if need be, though I would probably stop well short of the whole thing :)

Ravel's Bolero?

Sorry, forgot the code [Darren No. 19] Rentaghost? [Rab No. 20] Ravel's Bolero?

[Bigsmith] No, although my rendition is not perfect (done from memory).

There might be more plang-chongs in the intro.

[Bigsmith] Correct! Number 19 is, indeed, Rentaghost.

[rab] It's not the opening from Handel's Messiah is it?

Update:
1. Bob the dog - ISIHAC Theme 2. Chalky - Coronation Street 3. HB - Some Mothers Do 'Ave 'Em 4. Raak - Thunderbirds 5. Kim - The Archers' Theme 6. rab - Roobard and Custard 7. Bob the dog - Pomp and Curcumstance, Edward Elgar 8. Lib - Blind Date 9. ZK - Dangermouse 10. penelope - Blue Peter 11. Watty - Porridge 12. Kim - Tom & Jerry [Fred Quimby version] 13. Herr Bratsche - Monty Python 14. Raak - Star Trek 15. Bob the dog - Dr Who 16. rab - Have I Got News For You 17. Darren - Bergerac 18. Chalky - The Sweeney 19. Darren - Rentaghost 20. rab's plang-chonger - ???

[Darren] Nope. A hint?