I just wanted to reply because your post struck a chord with me. I had non-id twins (boy/girl) at 27+6, Twin 1 (girl) was 2lb 6oz and Twin 2(boy) was 2lb 6.5oz. He was behind right from the start - he had to go on a ventilator (she didn't need any help with breathing at all), in the very early days, he was always on a lower amount of milk, spent more time on cpap, was tube fed for much longer etc but once we got them home, he was a better feeder, put more weight on etc. However developmentally, he was behind his sister with sitting up, rolling (think there was about 6 weeks between them for this) and especially his speech. We were referred for hearing tests right from the start which were hit and miss - they said he couldn't hear very quiet sounds / had glue ear / just didn't co-operate with the tests. He made noises but didn't really speak, one day he said strawberry completely out of the blue but would then go for days without saying a proper word. I also went through a phase of thinking there was a real problem with his hearing but think now with the benefit of hindsight that he just couldn't be bothered listening to me etc!
To cut a long story short, they're now 6. He was seen by the audiologist until he was 5 I think to monitor things and at that point, he understood the test / co-operation etc. They say his hearing is fine, certainly not at a level where his development would be affected.
From a talking point of view, just after his 3rd birthday we were referred to a speech therapist because he had a slight stutter - obviously this is not your issue but the speech therapist's advice was quite good. She said that twins will develop at different speeds - as any 2 children would - and sometimes one twin is more vocal than the other reinforcing the problem as they tend to do the talking for the pair. Certainly as mine developed there was no 'need' for my boy to speak, my little girl always got in there first!! I do think too (although the speech therapist didn't confirm it) that premature babies can be behind in their speech - we were under the neonatal consultant as out patients until they were 2 - I definitely remember him signing them off at 2 saying he didn't need to see them again even though he said they were 3 months behind their peers in terms of speech. He didn't think that was unduly concerning at that age.
My advice would probably be to speak to your HV again - I think speech therapist referrals can't be done before a certain age - in our area it was 3, so keep an eye on things until then, do everything you can to encourage speech in the interim and then ask for a referral if you're still concerned in a few months.