the_constructor wrote:

On eBay I can purchased 20ltrs for £38 of trade masonry paint.

You get what you pay for. Check through the feedback for that seller
where some comments include "the paint was a bit thin and pale".

I use Johnstones Masonry Paint which is a trade brand, £40 for 10
litres. The stuff you are looking at may be OK, but what feeback there
is seems to suggest that it's what one would expect for cheap paint -
you're going to need more coats and it may not last long.