If your 964RS is one of several vehicles that you own, it may be worth trying Footman James as they do multi vehicle policies. They can combine classic cars and modern on one policy and provide guaranteed valuation on the classic cars (964RS is treated as a classic) - Tel 0845 458 6792 -ask for Margaret
For example my policy has:
Alpina B10 - daily hack
964RS - with guaranteed value of £70k
MK1 Escort Mexico (highly modified) with guaranteed value of £13k
There are no mileage limitations on the policy, cover is fully comp. and includes commuting in any of the vehicles.
All of this for less than £1k!! - if I insured the vehicles separately the best quote was £1,500!!!, strangely the Mexico is the most difficult to insure - not the 964RS.....