Tinacros, I have topped up oral HRT with extra oestrogen in the past and I didn't have any problems as far as womb lining build up is concerned.
I was using Angeliq (a continuous combined oral form of HRT) but the oestrogen dose was too low. I decided to top it up with a 50mcg oestrogen patch and it was somewhat better but still too low. My gynaecologist wasn't crazy about the idea but confirmed my own thoughts which were the worst thing that could happen would be a bleed out. As it happens, I didn't bleed and the womb lining was well below 5mm.
Interestingly, the US form of Angeliq has a lower dose of progesterone than the European brand so my ratio of oestrogen to progesterone was probably very similar to that.
It's not an exact science but generally speaking, if you are taking progesterone every day but your dose of oestrogen is too high, you will bleed but if you were not using progesterone every day then you might get womb lining build up.
Obviously it would be better to simply take a higher dose of Femoston but this is not a great time to get an appointment with your GP.