Getting a decent 2nd Sweet Corn cob?

Living in Scotland it's a bit iffy growing Sweet Corn outside unless we get a good summer (not 1 this year!) The Sweet Corn I am growing in the greenhouse borders (Swift) has been gorgeous, so sweet and tender very few have even made the kitchen having been scoffed straight off the plant. The 1st corms have been very good and almost 100% germination as the plants got shaken A LOT when the flowers were out but the 2nd cobs are small and poor germination. I think the germination problem is because I sowed all the seeds at the same time so all the flowers were passed before the 2nd cobs appeared so no pollen? but even if I had had flowers around for when the 2nd cobs appeared they would still have been very small. Is there anything I can do to encourage 2nd/3rd cobs to be of similar size to the 1st ones? The Sweet Corn look as though they are going to produce more flowers any minute but wondering if I might just as well pull up the plants now and make space to plant say a latish crop of Mange Tout in the greenhouse border which has worked quite well before. Sure some knowledgeable person will be able to give me good advice. I still have L plates on when it comes to veg growing and it probably shows!