Alex;
I wouldn't normally mention spelling or grammar but in this case I think that much of the meaning of your question has been lost because of it, so please forgive me if I misunderstood the problem.
You say that the BookingNumber is in text format, then ask if it is possible to convert BLAH1234 (your booking number) into text ... but you've already got it in text format; as far as I know the only way to store the value BLAH1234 is as text (or memo which is just a form of text) because it contains at least one non-numeric value. Consequently it is not possible to store the value as number only.
I'm going to hazard a guess here and presume that what you would like to do is to be able to search on part of a booking number, rather than the whole number. This can be achieved by using a wildcard "*" in the search criteria; the asterisk tells the query to allow any number of characters of any value to be present in the field.
First, let's assume that the text to be searched for is in a textbox called txtSearchValue on your form. For the BookingNumber field in your query use the following for the criteria:
LIKE "*" & [Forms]![frmFormName]![txtSearchValue] & "*"
Where you replace frmFormName with the name of your form.
The above criteria will return all bookings if the search value text box is left blank. If you only enter one character, say "2", into the search value text box then all bookings with a 2 in, no matter where in the booking number it appears, will be returned.
I will, however, caution you about using a search like this by itself. You will need to ensure that a customer only has access to the their own bookings and to do this you will need to either validate the customer prior to carrying out the search (in which case you could just list all of their bookings) or to require additional customer validation information in the query. Of course if this particular query is for use only by a system operator then you might want to have full search functionality (but will still need to validate the customer prior to giving out or changing information).
Hope this helps.
Tim
PS: An easy way to build the criteria for a query is to click on the Build icon (a wand with three stars and elipises) on the Query Design toolbar. This will then allow you to select the form and controls on the form, thus ensuring that the correct format is used to specify the criteria.