@SimonM Generally speaking, you should avoid Weebly spacers whenever possible particularly for adding columnar spacing. (For horizontal spacing, use the separator element instead and turn off the line display; the separator allows you to set the pixel spacing above and below the separator so you know exactly how much space you're adding; the spacer element on the other hand tells you nothing about the amount of spacing beyond what your eyes tell you.)
With regard to your contact form, you'll need a little bit of css code on the page containing the form to center it. In the Weebly Site Editor, click on "Pages" and then click the desired page in the pages stack in the left-hand sidebar. Click "SEO Settings" in the left-hand sidebar. Scroll down to the "Header Code" box and insert the code below. In order to use the "margin" left/right auto attribute (which centers an element horizontally), you have to give the element itself a width. In the example below, I've set the maximum width of the contact form container to 400px, however, you can modify this number as makes sense visually for your page and form.
<style>
.wsite-form-container {
max-width: 400px;
margin: 0 auto;
}
</style>
