I wouldn't worry too much about it - the flowers will likely just drape down from the top. This is an old picture (2010) but shows a group 3 that hadn't been pruned hard in 7 years! I took this picture shortly before we cut everything off the arbour in order to remove the beautiful but vicious (those thorns!!) New Dawn rose that shared the arbour with it... (The odd angle in the photo is because DH took the picture while standing on a stepladder so he could see the top of the arbour too....)
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I like the 'abundant' look so don't do the recommended hard pruning - I just take off obviously dead stuff and maybe neaten up a bit. This is the same arbour from the other side last summer:
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And the top of the same arbour, viewed from the pantry window last summer!
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Don't prune now - too late in the season and you can lose flowers. Hard pruning of type 3 or late flowering clematis is recommended for this very reason - you will often wind up with a lot of heavy top growth on a very bare lower vine. Heavy pruning often keeps the vines more manageable as well improving their looks.

Of course there is no rule that you "have" to prune - these are just guidelines. But hard pruning the first couple of seasons is highly recommended and not just for type 3's but for any clematis. This helps them to establish well, to become more full with multiple stems and to develop a strong root system. It also can make more resistant to clematis wilt those varieties that tend to be prone that problem..

Thanks folks. I'll just leave it then.

I actually did prune it in the spring to about 12 inches.