No worries, Gloria
I don't know what happens when you make a mistake on a GRO cert request because I've never made one (touches wood!) but hopefully someone here will know. They charge a lot more if they have to do a search, but if you've got the name, district volume and page right it may be OK.
I found the Thomas family on the 1939 register which is available on FindMyPast but the only children there were Annie and Jack - the others were older and would have left home, two known to have married and found, but significantly no Bert. Annie was given two future married names, first Clarke and then that of the gentleman who may still be alive.
Then it was a case of trying to find those marriages on FreeBMD, especially as they were known to be local to Warwickshire and possibly Coventry, and then the children of those marriages using both surnames and leaving the birth blank.
I made too many assumptions when saying there were three sons of William Clarke and Nancy (Annie) Thomas because the surnames are too common, even if you limit the search to Coventry. However, the first birth matches the person you mention. I see now that the other two are rather close to Nancy's 2nd marriage so probably to another couple. It shows that you shouldn't get too carried away.
We're quite strict about not naming living people so I don't name Annie's 2nd husband, especially now as you've named his daughters (I think it's a fair assumption that William Clarke is dead). What's significant is that the marriage in 1950 in Coventry is for both Annie Clarke and Annie Thomas marrying the same man - of course the same person under both her names - but her children all have the mother's maiden name Thomas. I found the two you mention plus Jane 1958 who sadly died as a baby.
I think you'll need either Ancestry or FindMyPast soon enough unless they're available at your local library. Look out for free weekends, one month trials and special offers - Rootschat members usually post updates when these happen.
There are too many marriages for Jack Thomas and Mary Thomas so you'll have to ask relatives again. I'm pursuing a hunch on Bert, I'll post if I find anything.