@Bobby De La Rue
You think you have to be aware of the Classis forgeries and Belgian Pre 1940's high values. I tell you you schould watch out for the 1914-1918 Forgeries in the Marude and Albert the I, 90 % of the stamps is false.
Now you can go to Ebay and have a look at what they are sold for, I bet they have been sold for 20-50 times the value than the two you mention, you always need to know what type of stamp you buy, if the seller sells them as Genuine what would you think?
Well lets have a look. This stamp I show is typically sold for 3 to 60$ for the serie of 3, the thing is how many stamp collectors know the 3 stamps in this serie for most parts is false, if you find a Genuien I think it would be word investing in.

False stamp

Buy me as Genuine or ? What if think I sell it as a colour proof would you buy it ?

This stamp is MNH how much will you pay me for this one, not much I hope it is False if you know what to look for, go to ebay they are sold as Genuine I have for a fortune :-)
I could make it easy for you to see what to look for, but then again then I wont be able to sell this block of four :-)
How about a nice block of four? 10 C Known as a Dangerous forgerie you have to measure the highed of the stamp these are 0,2 mm shorter than the Genuine

Or 5 C stribe of 3 misperforatede or sold as Proofs, they are all false.

Remember you can always buy any of these stamp in any Auction house in the world and they will be sold as Genuine, Proofs or essay what ever name you can come up with, remember approximately 90 % is false.