Hello , I was wondering if anyone would know the size of the spring that tensions the clutch pedal.

Our car, a 1939 Railton Six with a 3 speed stick shift didn't have the spring fitted when we acquired it.
I have ordered a generic tension spring from ebay .
The holes the spring clips in to are 5 1/2 inches apart, I have ordered a 5" spring , http://www.ebay.co.uk/itm/331979023425?_trksid=p2060353.m2749.l2649&var=541025669644&ssPageName=STRK%3AMEBIDX%3AIT but are unsure if this will give enough tension.
We had a new clutch fitted last year by a local garage but are still struggling to stop the clutch juddering in first (low) and reverse gears.
Also when the car is warm I cannot engage low gear, so I have to turn the ignition off shift into low gear , start the car and move off before the cars behind get too grumpy.
I have just read Doug's reply to Geoff whom has the same problem, so I thought I would start with the spring.
Many thanks in advance.
Regards
Russell