Winterbotham's book has some errors because he was forced to write it completely from memory, Brit govt having denied him access to official docs of the time. His assertion that at no time did MI consider sharing Ultra intel with the Russians has never been contested, not even by MI. His book is notable because it opened the floodgates for other works about Ulta, most notably Bennett's authoritative work with which you're no doubt familiar. Importance of Ultra in Western Allied strategy is either absent from or completely downplayed in wiki milhist articles. When I did submit a reliable Bennett reference (journal article "Ultra and some command decisions") with sourced page numbers etc to one milhist article Western Betrayal some time ago, it was promptly thrown out by consensus as "commie propaganda". That kind of censorship by a reactionary clique has got to change. An uphill task.